Many things bring on homework: upcoming tests, grades, main ideas that must be learned. These are fair reasons to assign assignments, because they teach you new abilities and let you practice certain skills. With good reasons, there are unfair reasons as well. Teachers who have nothing better to do, students misbehaving, and students not reaching the desired stopping point in the teacher's schedule. These causes put an abnormal amount of stress on the students.
First, let's focus on the good reasons. Homework can be a valuable tool in the teacher's hands, but only if they know how to wield that tool properly. Giving students 2 hours of boring homework will not encourage the student to learn. They will just skim through, filling in the answers. On the other hand, if a teacher gives a one hour assignment that interesting and fun, then the student will be more enthusiastic about it, therefore absorbing the assignment instead of skimming over it.
Homework allows the teacher additional time to teach the student. If certain skills are needed later on in life (or at least on the SAT exam), the homework would allow more time for the student to remember the skill. If there is an upcoming test (finals, SAT, etc.), assigning homework is a great way for the teacher make sure the student studies.
With all of these benefits, there have to be some drawbacks. Teachers who assign a lot of homework, especially when it is due the next day, are feared. Students are stressed and can't go into that teacher's room without feeling dread at how long tonight's assignment is going to take. One hour, two, or more? They wait in fear for the announcement of the homework assignment. After a grueling night of studying and writing, the students barely have enough time to sleep. They sleep in