It wasn't that important for me. I never did it but I always got an A on my tests. But I guess, if you have trouble with it in class or have a hard time paying attention it could help you learn the lesson better... :-/
Because then you earn a high grade, which will allow you to get into a better university, and hopefully result in getting a good job. Teachers give homework because it allows us to practice what they've already taught us in the class, or continue learning about it at home, because there's only so much time in the day that it can't all be squeezed into less than an hour a day. BTW, English and math are two of the four core subjects, in fact two of the most important core subjects, so it's important that you have A's in THOSE classes. An A in PE or art doesn't really help you getting into college, aside from bumping up your GPA.