Homework's largely supposed to be about reinforcing the lesson for math courses. For other courses it's largely about reading which shouldn't be done in class. Tests/essays are for assessment.
The amount of hw students get in the us is astounding. It's not always productive either (more often that not it's not). And it seems like there's no pushback against it.
2. It mimics the real world where sitting your butt in a chair and turning in useless reports is valued -- perhaps not as much as actual outcomes, but still valued. Perhaps that's stupid, but in most jobs it's a reality. IMO this isn't a great motivator, but it's one a lot of teachers cite.
3. Helps pad the grades of students who are terrible test takers / have anxiety issues.
4. Students are... students. They don't always know what's best for themselves. Making homework worth a portion of the grade communicates to students that they need to practice. Patronizing? Maybe. But also a reality.
5. It's the easiest way to inflate grades without explicitly lowering standards.