Because they need to put their feet up. Their legs and feet are simply not strong enough to support these functions for great lengths of time. For this very reason, bats, when in their bat caves, or roosting in trees, take the weight off of their tired limbs by hanging upside down. Their breast-feeding babes, born live, as are all mammal offspring, have no choice but to hang in limbo with mom. Together, they reap the benefits of downward gravitational pull.
Bats also invert our common beliefs regarding their eyesight.
One would be incorrect in assuming that the bat has keen night vision, to assist it in its feeding frenzy. The bat employs an echo system, whereby it tosses out a high-frequency sound, one inaubible to the human ear, which in turn strikes a target, and then bounces back to the bat, thereby letting it know what lies ahead. Were it not for the bat's radar, we would hear even more bumps in the night than we already do.