You're missing the most basic utility they provide... that of making sure other code is calling the function with the right types of arguments. That's a lot of coverage over a language without a compile type checked type system.
That's not a utility in itself, it depends on what the types represent to know if this is a useful property or not. For example, a Cfunction which is declared as "void foo(int a)" does ensure that it's called with an int, but if it's body then does "100/a", calling it as foo(0) is allowed by the compiler but will fail at runtime. It's true that that the equivalent Python function (def foo(a)) can fail at runtime when called as foo(0), but also foo("ABC"), but it's a matter of degrees, not kind.
