What effect of the sweeping in curling?

Answer 1

The friction from sweeping heats the ice slightly and reduces the friction between the rock and the ice. This does two things; it makes the rock travel farther, and it also makes the rock travel straighter. When a rock is released, depending on the type of shot being thrown, the players will judge whether sweeping will help or hurt the trajectory of the rock, and decide whether the rock should be swept or not. Usually, the sweepers will pay attention to whether the rock might need sweeping to travel farther, while the skip pays attention to whether the rock might need sweeping to travel straighter, and they communicate this with each other while the rock travels down the ice.

Answer 2

Sweeping makes the ice more slippery, making it possible to do mid-course adjustments to the speed and trajectory of the stone.