Ordinarily, if we leave our pis on overnight or not, we leave them idle, not running anything.
What could we run then? We could use our CPU to help with science, mine bitcoins, seed torrents, and lots more?
When your pis not doing anything else, what do you leave it doing? I connect it to BOINC and run [email protected] and a couple of other projects on there, helping science and stuff. In the past I've used it to seed some torrents that not many or noone seed, just keeping them alive if I've found them useful. A few people run bitcoin miners, to do that effectively you need to buy special equipment really, though. I've also used it to compile things or crack passcode hashes, that sort of thing.
Don't worry, I'm only testing my own website's data to keep users secure.