There was a little more exposition in the film's original screenplay. In short, the Dark Star (and other ships of the same class) were explorer vessels charged with identifying "rogue worlds" that had the potential to interfere with future colonisation efforts.
It is the mid 22nd Century. Mankind
has explored the boundaries of his
own solar system, and now he reaches
out to the endless interstellar
distances of the universe. He moves
away from his own small planetary
system in huge hyperdrive starships:
closed-system spacecraft that travel
at mind-staggering post-light
velocities. Man has begun to spread
among the stars.
embark with generations of colonists
searching the depths of space for
new earths, now homes, new
beginnings. Far in advance of these
colony ships goes a new pioneer: the
scouts, the pathfinders, a special
breed of man who has dedicated his
life to blazing the trail through
the most distant, unexplored
galaxies, opening up the farthest
frontiers of space. These are the
men of the Advance Exploration
Corps. The task they face is one of
unbelievable isolation and
loneliness. So far from home that
Earth is no longer even a point of
light in the sky, they must comb the
universe for those unstable planets
whose existence poses a threat to
the peaceful colonists that follow.
They must find these rogue planets
-- and destroy them. Among these
commandos are the men of the
scoutship Dark Star.
What sort of instabilities were they worried about? Mostly orbital irregularities, apparently.
BOILER: Hey, Doolittle, here's one. An unstable planet. 85% probability of an unstable planet in the Veil Nebula that will
probably go off its orbit and hit a star.
BOILER: There she is. Definite 99%-plus probability that the planet is going to deviate from its normal orbit in another twelve
thousand rotations. It'll spiral in toward its sun, and --
PINBACK: Eventual supernova.
DOOLITTLE: Good stuff. Let's vaporize it.