On quora, which is an interesting if not always satisfying site where people ask questions and others propose answers, someone asked is school homework should be abolished. This is my (modified) answer
"Educational theorists" are infamous amongst actual teachers, for the bizarre courses of action they advocate - like this one. It's often because they have not actually taught much, and have sailed through the educational system from K12 through college to graduate degree without experiencing much of the real world.
I recall particularly the "expert" who said that deadlines should be abolished in school, that it didn't matter *when* the students did the work as long as they did it. Not only does this fly in the face of the real world, where you have to meet deadlines, it ignored the impossible situation this would put teachers in, of grading an enormous quantity of stuff at the end of the class (when there really was a deadline). It also assumes that the teacher's grading can do nothing to help the student improve (there will be no time for the student to improve if everything is handed in very late in the class). Yet this idiot charged $8,000 a day as a consultant, according to the newspaper report.
Homework for homework's sake should not exist. Homework to help a student learn about the topic, and to learn discipline, is vital.
In one sense, if you ask why students do homework, you may as well ask why students do any work at all? What is the difference between doing something in class, and doing something at home?
The trend has been that students spend much less time on a class, outside of class. A problem with homework is that you don't know who actually does the work; in K12 so many parents get involved now, and group activity is common (when it isn't supposed to be), so it becomes difficult to know whether the student is learning anything him or her self, and even more difficult to make a formal assessment. Formal assessment is crowding out actual learning as it is, thanks in part to government regulation, in part to people who think teaching is only conveying information (which is a minor part of good teaching).
A major purpose of homework in the past was to have students read something that would convey information, so that the class could concentrate on other things requiring more thought during class time. But many university classes, with hundreds of students in the class, now amount to oral books where the instructor does not know the students and is very limited in interaction with them. Further, the habit of reading to learn is going away ("tl;dr": "too long; didn't read"), people want to hear-see-etc. So even if reading is assigned as homework, a great many students will not bother (some, in college, won't even obtain a copy of a class textbook, whether legally or not). In this respect, homework may appear to be useless simply because so many students won't do it.
All disciplines require practice. Virtually no one does something non-trivial once, and then does it well. A reason for homework is to give students additional practice time. Some subjects are not as amenable to practice as others; some require practice in groups; some can be practiced individually.
The question is, can the student learn whatever it is they're supposed to learn, using only class time? This varies a great deal with expectations. In the USA, expectations have been drastically reduced in K12, where students memorize answers and regurgitate them on a standardized test at the end of a class or year. Where higher standards are maintained, where students are expected to understand, not merely to memorize, to be able to think, not merely to regurgitate, more time is needed.
So the amount of homework that is reasonable depends on expectations and on willingness and capability of students to learn - learning is not passive, and some students are going to learn quicker, some slower. It is likely, given the recruiting process, that students at (say) Duke are going to put more effort into a class than students at a community college, not only because of their backgrounds but because of their financial and family situations. The ultimate question is, is there time to do all that needs to be done, during class time. The answer, as with most things, is it depends.
"The worst form of inequality is to try to make unequal things equal." -- Aristotle
"Always do right--this will gratify some and astonish the rest."Mark Twain
"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away." Antoine de Saint-Exup'ery
"Not everything that can be counted counts, and not everything that counts can be counted." Albert Einstein
"Make everything as simple as possible, but not simpler." Albert Einstein