Well, being primates, I imagine if humans had tails by default, they would have the same sort of prehensile tail as many other primates. This is one of those weird things that enables hybrid types like Skunk Boy and Tex to work in-universe, humans still have a bit of "tail code" left in their genetic makeup, that gets activated by whatever they are hybridized with, giving them the appropriate animal tail in most instances rather than a monkey tail.

As for Domec, and many of the other creatures I draw that aren't directly based on anything in the real world, I haven't given much thought to the tails or what functions they might serve outside of my personal aesthetic tastes. Domec's kind are descended from the Lucifer, and I haven't decided what their tails do either, aside from possibly being useful in combat somehow, and perhaps having some limited prehensile capability. Perhaps in a low-gravity environment, the tails could be used to help them move about either by acting as a rudder of sorts, or by moving their tails similar to how aquatic animals do, or even to push off of stationary objects to help them better position themselves or remain steady while working.
Of course, tails are also useful for communication, as they can play a huge role in body language--I'm certain lifting one's tail while bending over is a pretty universal invitation to a breeding partner, for example!