I got very worked up over the idea of homework as a teenager. If I know the material and can show it why should I have to do this drudge work that only gets checked for completion anyway yadda yadda. Acing tests without doing the work became kind of a point of pride, which seems to be common among honors underachievers.
But 90%+ of the time getting students to learn will require them engaging the material outside of class, whether that means repeating types of calculations or reading the novel they've been assigned. If homework doesn't factor in to the final grade, I can't imagine how teachers get the students to actually do the reading.
Besides all that, homework should impart some semblance of responsibility and organizational skills, which aren't to be sniffed at.