It is often used for builtins, because emulating the default Python behaviour of accepting arguments both by position and by name is a pain with the Python/C API. (There are other use cases for positional-only arguments, such as accepting an arbitrary function and an arbitrary set of arguments to call it with at the same time—for example, to invoke it in a new coroutine—but they are pretty rare.) This pecularity of most builtin functions has been there since before Python 3 was a thing, it’s just been undocumented and difficult to emulate in Python before this syntax was introduced.

As for unexplained noise—well, all other parts of the function declaration syntax aren’t explained either. You’re expected to know the function declaration syntax in order to read help on individual function declarations; that’s what the syntax reference is for.