Why we might want to do it
I think it's fair to say I'm addicted to using GitHub. I've used it so much in the last couple of years that I don't understand/remember how we got any serious collaborative coding done before. In particular, the ability to comment on code line-by-line, having conversations, updating the pull requests, and merging them with a single click is in my mind so much more rewarding and productive than having to comment on a patch in an email discussion.
However, I occasionally want to do a full review of a package that someone else has written, and comment on various parts of the code. While it is possible to leave line-by-line comments on a commit-by-commit basis, GitHub does not provide an official way to review the latest full version of a file or package.