Makes changing water less of a chore. You hook it up to your sink. Turn on the water. Through a ventury effect it will start a siphon from your tank into the sink. If the faucet is low enough. If the faucet is not high enough you just keep the water running. There is a gravel vac attachment that you can install. Then after you empty enough water from your tank you can fill it back up.
True you have to treat the water after its in the tank. But, I have not had a problem doing up to a 50% water change on a 55 gallon tank if I use a water conditioner before I fill up the tank. I use Prime by Seachem, by the way.