Why should student have homework?

when I was growing up we didn't have much homework we had allot of time to be kids. My 14 year old sister goes to school and when she comes home she has an extra 4 hours of homework, that to me is unfair. Why should they even have homework when school was made for learning. School is for learning and your home is home. Teachers are paid to teach and they have 8 hours to teach. And worst is also the way they treat these students when they don't do the homework, my sister got 8 Fs for not doing 2 homework. why make students put all the effort? Kids need more time to be kids and teachers have 8 hours to teach what they have to, so why give students more work!