Normally you do it to make closures:
def make_adder(x):
def add(y):
return x + y
return add
plus5 = make_adder(5)
print(plus5(12)) # prints 17
Inner functions can access variables from the enclosing scope (in this case, the local variable x). If you’re not accessing any variables from the enclosing scope, they’re really just ordinary functions with a different scope.