Provide an incentive for students to do the thing they should be doing anyway.
Give an opportunity to provide feedback on the assignment.
It is totally useless as an evaluation mechanic, because of course the students that want to can just cheat. It’s usually pretty small, right? IIRC when I did tutoring we only gave like 10-20% for the aggregate homework grade.
Realistically I think the more common case is to think you know the material, skip studying, and then faceplant on the test. Homework should help self-correct.
But yeah, I could it being annoying if you really do already know the material.
IMO the ideal class would be 4 or so students working together on a bespoke project, with weekly check-ins with some grad student teaching assistant. The goal would be to do something interesting and new. Of course nobody ever has enough teaching staff for that kind of thing.
Before college, when I was a kid, I just had the textbooks, so I read the chapters and did the assignments… it was much better than sitting and listening in lectures, then doing some small assigned subset of the problems…