Well, the only reason to turn it off would be to save energy. That's a reasonable reason. Personally, I leave my pc doing folding@home all night, so I effectively donate its utility cost to charity, but doing something like that will eat into your wallet. If you don't mind sparing a few dollars, I think folding@home (folding.stanford.edu) is a pretty good thing to do. Distributed supercomputing for disease research.
If you do mind the extra money, or you value the electricity more than the computing, definitely turn off at night.