This post is a response to a reader question.
Curling is a 400+ year-old sport about which I know very little. Wikipedia saves the day again.
Curling is played on level ice. However, droplets of water called pebbles are added which cause the stone (pictured above) to curl when it travels across the ice. According to Wikipedia, sweeping accomplishes two goals: to increase the distance the stone travels, and to change the amount of curl. The strategy of sweeping seems to involve figuring out when to sweep and how hard to sweep (apparently that has an effect on the amount of moisture created).
Overall, the point of sweeping seems to be to influence the curl of the stone by wearing down the pebbles.
Do you know more about curling than I do? If so, please contact me so that I can update this post appropriately.
Olympic Science: The Science of Curling at Fox News for a discussion of curling.
Does sweeping in curling achieve anything? at Metafilter for a bunch of people talking about sweeping in curling.
The Physics of Curling at the University of Toronto for an interactive description of how curling works.
Curling at Wikipedia