mypy: What is the need for covariant in this TypeVar?

U = TypeVar("U", covariant=True)

class IterableAndSized(Protocol[U], Sized, Iterable[U]): ...

I want to make a protocol as above that uses the Sized and Iterable abstract base classes to define an acceptable type; e.g.

T = TypeVar("T")

def tailvalue(index_from_rear: int, x: Iterable[T] | IterableAndSized[T]) -> T
    if isinstance(x, Sized):
        for i, val in enumerate(x):
            if i == len(x) - index_from_rear:
                 return val

    return list(x)[-index_from_rear]

this can help optimise retrieving values near the end of a generator expression.

Why do I need TypeVar("U", covariant=True) rather than just TypeVar("U") there? IIUC it means subtypes are permitted or something, I'm sure that's okay but I don't know why it's a requirement here