Why bother having a process for proposing changes to your open source project? Why not just let people do what they're doing and merge the features when they're ready? Well, you can certainly do that if you're the only person on the project. Or maybe if it's just you and a few friends.
But if the project is large, you might need to coordinate how some of the changes land. Or, at the very least, let people know a change is coming so they can adjust if it affects the parts they work on