In later elementary school at least (in Canada, in French immersion, in a moderately-above-average public school), homework was very effective in increasing knowledge. Our kids enjoyed it, and expanded their vocabulary and fluency in a second language. They had ample time to play, to participate in sports, and to sleep. I can't imagine them doing less homework, and I suspect they could have done much more.

The whole experience of learning is about practice. Early on this can be in well crafted exercises and simple homework.

I'm sure that homework can be counterproductive and poorly managed, but I've seen it succeed in moderation (2-3 hours daily).