Is it because parents expect it? Is it needed because the school hours are not long enough for all the learning to be done? Is it necessary to cement what has been learned in the classroom? Is it extra pressure on teachers because of SATS?
When I was in primary school (80s) it was literally just reading and times tables, which I get, these things underpin all their other learning. But my dc get weekly homework and always hated doing it, and I hated forcing them. Sometimes the tasks themselves seem pointless, and I always rolled my eyes at the model making tasks where kids would be lavished with praise or get extra golden time for something clearly constructed by their parents. I remember hearing one mother tell her child that she would carry the model Tudor house into the classroom as "I'm not risking you dropping it and ruining it" 
Why the increase? Or was I unusual in not having homework in primary school? In fact maybe I'm unusual in saying I'd be perfectly happy if my dc had no homework at all until
senior school!