It gives you a future market share via so many people using your software for free. They won't pay if it weren't free anyway, but would use a competing piece of software. That is, without the free (as in freedom) version, few would even consider paying for a commercial license.
Did you ever think why Microsoft is so lazy about enforcing the end-user Windows licenses on non-business customers? Maybe they want kids get used to to running Windows at home, and expect it in a work environment, and value this higher than the lost revenue from "pirates"?
