Because they need to put their feet up.
Since bats are the only mammals who can fly, with wingspans ranging from 6" to 6', they failed to make great strides in the walking-about, or in the standing departments. 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. The bat, a nocturnal creature, hunts its prey at night, and rests up (or down, as is the case) for the evening's events by day. One would be incorrect in assuming that the bat has keen night vision, to assist it in its feeding frenzy. Not so.
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.
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