I do it to keep my pants from getting caught in my chain.

Around 6 months ago I forgot to roll my pant leg up while I was on my fixed gear. My pant leg got caught in the chain, and since I couldn't stop pedaling it almost made me crash while riding on a very busy road. I'm assuming it wouldn't have been as big of a deal if I was on my geared bike.

Needless to say, I never forget to roll my pant legs up anymore. I don't wear socks that are high enough for me to tuck my pants into them.