Believe it or not, they don't smooth the ice for curling. They have this little contaption, it spits water up in the air and it instanly freezes on the ice when it lands, and it makes little bumps on the ice.
If the ice was perfectly smooth they wouldn't be able to spin the stone and hook it around other stones. How do I know this? Well, I'll admit it. About two years ago me and 3 of my buddies went to the RMU Island Sports Center here in Pittsburgh. It was the same night as the Cowboys/Panthers NFL playoff game to be exact.
Sweeping was arguably one of the most physically draining things I have ever done. I was down to a t-shirt by the end of the night and was drenched.
And the reason they sweep in front of the stone is to speed up the stone because it rounds down the little bumps on the ice. The friction of the broom on the ice also generates heat and that helps increase the speed of the stone also.
I know this stuff because they made us watch a video before we were allowed on the ice. Booze was not allowed by the way.