Prenatally , babies are very comfortable, oblivious of the external world within the comfort of their mother's womb !

At birth , they are expelled into the external world , which environment could be very inconvenient to the baby !

Crying is thus a communication reaction , by the baby , expressing displeasure about the unfamiliar and or inconducive environment!

Crying at birth is also indicative of the perfect state of the baby's nervous system. This is why mothers beats the new born to stimulate cry if they didn't do it Voluntarily!
I hope this helps our understanding why babies cry at birth and why mothers does beat the babies to stimulate crying!