There are Daily, Weekly and Monthly “cron” tasks that Unix runs overnight, if the machine is left on. Mostly cleanup of log files and other housekeeping, no harm done by their not being done, unless maybe you’re running an enterprise server or some such “big iron” application. There are many freeware and shareware Mac utilities that allow you to run these tasks manually; MacJanitor, Onyx, Cocktail, Macaroni, to name just a few, and of course they’re always accessible via Terminal. The issue of whether to leave machines on or off is debated incessantly on all computer forums, with neither camp ever making a good case for its side.