In the Marvel comics mainstream universe, magic is a real thing. People use it because it's there and because they can, just like any other ability or source of power.
The source of magic user's power depends on the magic user. Typically, there is no non-magical explanation for their powers. Magic, as a force, exists in the Marvel universe, and different characters have access to it in different ways. Certain scientifically-minded characters consider magic to be a science, just a kind of science we don't otherwise understand, but they never claim to have a "mundane" explanation for it using known science. (Reed Richards and Doctor Doom are two scientists who have studied magic; Doom is himself a sorcerer.)
In most cases, magic used by humans is actually drawn from other, extra-dimensional beings. Doctor Strange, for example, draws his magic primarily from the Vishanti, who are three elder god-like beings that grant him power in exchange for service. He has also, on occasion, drawn power from Dormammu (essentially a "demon") and other supernatural beings. Similarly, Scarlet Witch's Chaos Magic comes from the elder god Chthon.
There is also a level of ambient magical energy in each person, and in the universe as a whole. Certain characters can manipulate that energy. Scarlet Witch's mutation, for example, would allow her to manipulate the ambient magical energy even without her Chaos Magic, though she would be very significantly weaker. Adam Warlock and probably Ororo Munroe (Storm) can also use magic this way. (Adam Warlock's magic is amplified quite a bit by the Infinity Gem he possesses.)