Why do bats hang upside down from tree brances?

Bats hang upside down primarily to help them get in to flight ("take off") quickly. Unlike birds, they do not have strong wings that provide enough lift for taking off directly from the ground. Their hind legs are also not strong enough to run fast to get the necessary take off speed. When they hang upside down they can get in to flight right away just by releasing their grip. If a bat accidentally falls to the ground, it has to climb up to a tall spot such as a tree branch to take off. 

Bats also obtain additional advantages due to hanging upside down such as safe guarding themselves from predators. The unique ability to hang upside down from tree branches and on the inner roof of caves makes it difficult for predators to reach them. Newborn bat pups can cling on to the mother's body and get transported by her. Usually, they stay in the bat roost where several bats nurse the newborns. The muscle arrangement in their legs is such that when they hang, their claws are kept closed due to their body weight. This makes it easier for them to hang spending minimal energy. They have to use energy only to open the grip and fly. Their grip is so strong that if a bat dies while hanging, it will keep hanging until forcibly knocked off by another bat or something.

Source: thehindu.com