It has nothing to do with website design. It's part of the HTTP protocol. A poor part in today's understanding and use cases, but in the 90s it would have made sense.
That is also a form of basic auth, you still pass the credentials like "username:password", optionally base64 encoded but I don't believe that's required.
Edit: actually, after looking into it a bit more, it seems like passing credentials in the url will actually cause the browser to send it as an authorization header instead. So in essence it's doing the same thing.
