Function Annotations: PEP-3107

I ran across a snippet of code demonstrating Python3's function annotations. The concept is simple but I can't think of why these were implemented in Python3 or any good uses for them. Perhaps SO can enlighten me?

How it works:

def foo(a: 'x', b: 5 + 6, c: list) -> max(2, 9):
    ... function body ...

Everything following the colon after an argument is an 'annotation', and the information following the -> is an annotation for the function's return value.

foo.func_annotations would return a dictionary:

{'a': 'x',
 'b': 11,
 'c': list,
 'return': 9}

What's the significance of having this available?