Given that the physicist Kip Thorne played a major consulting role on this movie and even co-wrote the original script treatment, I would bet that the theory of time travel being assumed was the Novikov self-consistency principle, in which there is only a single self-consistent timeline, with no ability to "change" the past and no branching of the timelines. Thorne was the physicist who first discovered the theoretical possibility of traversable wormholes in Einstein's theory of general relativity, and the fact that the theory predicts they could be used for backwards time travel under certain conditions. After this discovery, Thorne and colleagues made the case that no matter what initial "boundary conditions" you set up with a time machine, it's always possible to find self-consistent outcomes. For example, in his book Black Holes and Time Warps he discusses the analysis in published papers like this one, which imagines that a billiard ball is sent towards one mouth of the wormhole on a trajectory that, if allowed to continue, will cause it to emerge out of the other mouth in the past, on a collision course with its own earlier self. This would be a potential paradox because if it knocked its earlier self away and prevented it from going into the wormhole, then it should not have emerged out of the other mouth in the past, and thus there would be nothing to deflect its earlier self and its earlier self would go into the wormhole and emerge on a collision course, etc (see the top part of the diagram below, which I found on this page).
The resolution found by Thorne and his colleagues was that the billiard ball might emerge from the other mouth on a slightly altered trajectory, which would still cause it to hit its earlier self but to only deliver a glancing blow that would slightly alter its earlier self's trajectory going into the wormhole rather than knock it away from the wormhole altogether, and that this slightly altered trajectory would be just the right one to cause it to emerge in the past on a trajectory to deliver that same glancing blow--thus there could be a single self-consistent sequence of events (see the middle part of the diagram).
Thorne and his colleagues argued that this is probably how it would always work with time travel scenarios, no matter how convoluted--in fact, they found reason to think that for any given initial trajectory of a billiard ball, there were likely to be an infinite number of different self-consistent continuations, so if time travel were really possible the universe might simply choose one at random (they also analyzed the problem in quantum physics and found that different self-consistent continuations should have different probabilities).
In a universe that respects the Novikov self-consistency principle it is still possible for time travelers to affect the past, perhaps even playing a decisive role in causing certain elements of their own past, even though it's impossible for them to change anything (so they can't eradicate themselves from having existed, no matter what they do). The book Time Machines by Paul Nahin has a section discussing this idea on p. 269 which gives a number of fictional examples, such as the story "The Past Master" by Robert Bloch, in which (spoilers!) a time traveler from the thirtieth century travels back to our own near future to save some artwork from a nuclear war he knows occurred at this time (but does not know the exact cause), and his time machine is mistaken for a Soviet weapon and ends up being the trigger for the nuclear war.
For an example of a time-traveling billiard ball playing a decisive role in causing itself to have gone back in time in the first place, look at the bottom part of the diagram above. There we see another self-consistent billiard ball scenario, one where a billiard ball was originally on a trajectory that wouldn't have caused it to fall into the wormhole at all, but then it gets hit by its future self in such a way as to knock it into the wormhole, becoming that same future self (this scenario was originally mentioned in the paper by Thorne et al. mentioned above, see Fig. 6 on p. 1083, illustrations b and c). This sort of thing is similar to what may have happened in Interstellar--the future beings' trip back to the past (or more precisely, their transmission of gravitational signals back into the past) is the very thing that sets their past selves (i.e. humanity) on a course to become those same future beings and make a trip into the past. We could make this billiard ball scenario even more closely analogous to Interstellar by imagining that the billiard ball's original trajectory was aimed directly at a bomb which would destroy the ball if they collided, so only because its future self appeared from the wormhole and deflected it was it able to avoid destruction, just like humanity only avoided extinction because of the intervention of our own future selves.
A term for this sort of scenario, where a time traveler's actions in the past cause effects that will later ensure that the same time traveler goes back to the past, is a predestination paradox, but it is not a true logical paradox, it's only paradoxical in the sense of being strange and counter-intuitive (i.e. the first definition of 'paradox' here, not the second). This is related to, but somewhat different from, a bootstrap paradox in which there is some object or information that seems to have no creator, like if a time traveler went back with a future edition of the complete works of Shakespeare, and a hack playwright named Shakespeare copied them down in his own hand and released them under his name. This isn't really a logical paradox either, but one might at least doubt that such things would be probable even in a universe with time travel obeying the Novikov principle. (In Black Holes and Time Warps Kip Thorne mentions that for billiard ball/wormhole scenarios where there are multiple self-consistent scenarios following from the same initial conditions, quantum principles can be used to assign different probabilities to different scenarios, but I don't know of any work applying this to cases where there are self-consistent scenarios involving both meaningful and meaningless information loops. It's possible such an analysis might show that bootstrap paradoxes involving meaningful information with no origin are possible but very improbable, in the same way that it's possible but very improbable that a monkey banging away randomly on a keyboard could produce the complete works of Shakespeare). But even if one is concerned about this issue of probability, I think there actually aren't any clear bootstrap paradoxes in Interstellar since the higher-dimensional beings didn't directly give us the correct quantum equations to understand gravity, rather they allowed Cooper and TARS to discover them by falling into the black hole, and then transmit them back in time. Likewise, when Cooper transmitted the coordinates of NASA to his younger self, he had to ask TARS to "feed me the coordinates of NASA in binary", rather than just remembering the coordinates he had received in the past and transmitting them again (which would be a bootstrap paradox akin to the Shakespeare example).
As for why they did it this way rather than simply giving us the information directly, we can only speculate. Maybe they knew that circumstances would tend to thwart attempts to create bootstrap paradoxes, just like how in a Novikov universe they would definitely thwart attempts to create genuine contradictions such as a time traveler trying to assassinate one of their own ancestors before they had children. But another possible answer could be that their minds had grown too different from ours for them to know how to communicate with us directly rather than helping us to figure out how best to communicate with each other (with the movie emphasizing that our ability to communicate, and our intuition for the best person to try to communicate with, is grounded in love), as suggested by these lines from the screenplay:
COOPER: 'They' have access to infinite time, infinite space...but no
way to find what they need — but I can find Murph and find a way to
tell her — like I found this moment —
COOPER: Love, Tars. Love — just like Brand said — that's how we find
Based on all this, I don't agree with the answer to the other question, saying that "Humanity survived through the plan B"--the person asking the question (and the person giving that answer) seems to be assuming a framework in which time travel does change the past, so there must have been some "original" unaltered history which gave rise to advanced beings who sent signals back in time, creating a "new" version of history that we saw in the movie. But if you think in these terms, in the original version of history there wouldn't have been a wormhole, since the wormhole itself was constructed by the future descendants of humanity, see this discussion of traversable wormholes by Kip Thorne in Ch. 14 of The Science of Interstellar:
I doubt very much that they can form naturally in the astrophysical
universe. My only real hope for forming them is artificially, in the
hands of an ultra-advanced civilization. But we are extremely ignorant
of how such a civilization could do it ... In Interstellar, however,
the wormhole is thought to have been made, held open, and placed near
Saturn by a civilization that lives in the bulk, a civilization whose
beings have four space dimensions, like the bulk.
So if you assume an "original" timeline with no intervention from our future selves, Plan B couldn't have worked because there are no other habitable worlds in our solar system to settle, and travel to another star system would take many thousands of years with near-future technology. In a single self-consistent history respecting the Novikov self-consistency principle, the timeline we saw onscreen was the original one, so the bulk beings could be the descendants of the very same humans we saw at the end of the movie, perhaps descended from both the people who had escaped Earth under plan A, and the children grown from the embryos Amelia Brand had probably started growing according to plan B.
If this still seems confusing or paradoxical, you might take a look at my answer here which tries to make more sense of where self-consistent causal loops like this "come from", by imagining how we might simulate a universe where time travel works this way on a computer.