Because you can pinpoint the "hotspots" where your circulation tends to leave all the debris, so you can easily siphon the top layer of the sand and be done. Also for a really deep cleaning just mix all the sand up with your hand, let settle for ~20 minutes, and suck up the debris that stay on top, also doing this the filter will get a lot of it. Just rinse your filter after you do this. Also if you get the right fish, the only thing you will ever be cleaning up is poop or other undedible debris. I have virtually 0 uneaten food in my sand tanks.