If you're going to use your computer on and off throughout the day there is nothing wrong with leaving it on. In fact, it's actually harder on the equipment to turn it on and off frequently than to just leave it running.

The rapid heating up and cooling down of turning on and turning off frequently can cause damage. It’s better to just leave the machine on for a while in case you will need it again.

Leaving the PC on over night isn't going to hurt it either, although for the sake of saving electricity it may not be a bad idea to shut it down when you go to bed. :)

It's still a good idea to shut down your PC when it won't be used for a long period of time, but leaving it up and running in most cases won't hurt.