If you have never contributed to a project before, the "attach patch and upload/send mail" part is a significant hurdle. You have to figure out which mailing list to use, which is not always easy to find, then find out the person/group to address it to. That alone already discouraged many people from even bother.

It's hard when you have lots of folks jumping into a code review, and you don't want to duplicate effort or you want to rapidly iterate on your code.

You know those email chains that just continue to fork? Where people reply to the original (or the first couple replies) with their wall of text after a couple other replies have already trickled through?

That's what a collaborative, stateful PR solves.