There are a few kinds of PRs out there in the wild. There are the PRs that are trivial to review, they are small, self-contained, etc.

There are others though:

• You might be updating a function signature in a library and have to update the call site across hundreds of files. There's too much noise to find the parts that are most important.
• You might be refactoring something more involved and want to tell a story with your PR, or it's just an idea and you want to walk through it.
• You might have left a lot of comments annotating your PR but how do you make sure that folks read them all in the right order? They're going to look at the diff and read it top to bottom anyway.