Curling brooms create a buffer for the rocks (especially the brooms used today). Because between the Ice and the curling rock there is friction although less than a regular floor the rock eventually slows down. To produce less friction between the rock and the ice sweeping very close to the stone in front of the sliding path buffs the ice creating less friction. When this happens the rock will travel farther than if it wasn't sweeped. Remember curlers put a spin on rocks and because there is less friction between the rock and the ice when sweeping the rock will curl less (run straighter) than when not swept.
However this is true for most of the time the rock is spending in motion sweeping in front of the rock just before it stops will make the rock curl more. Watch the olympics and when you see a team put up a guard and the guard needs to curl more to be effective they will wait till just before the hogline to sweep the rock really hard and make it move sideways more.
Oh, I see now ............
