Rationale behind Swift's BidirectionalCollection extension

Question

It is my understanding that extensions are often used on protocols to provide a default implementation. My question is about a pattern I have observed in the Swift Standard Library's protocol BidirectionalCollection. Consider the code below:

protocol BidirectionalCollection: {
    // other method requirements ...

    func distance(from start: Self.Index, to end: Self.Index) -> Int

    // other method requirements ...
}

followed by

extension BidirectionalCollection {
    // other default implementations ...

    @inlinable public func distance(from start: Self.Index, to end: Self.Index) -> Int

    // other default implementations ...
}

I don't understand two things about this extension:

1. Why we are able to write the method without a body (which I have always seen in default method implementations), and

2. What value this specific extension brings. At first glance the only difference just seems the leading @inlinable public. But can't protocol method requirements be prepended with those keywords anyway? What is the use of doing it in the protocol extension?