If you are going to be using it again in the next couple of hours leave it on, otherwise shut it down.
In desktop computers, there are 2 to 4 fans as well as the hard drive, all of which have motors. In a notebook computer, there is very likely one fan and the hard drive. These motors will wear out and as they wear they will make more and more noise until they fail. If you have been running yours 24×7 and it is about 3 years old, it was probably virtually silent when it was new and now is beginning to make an annoying sound.
Computers normally are set to hibernate after n minutes (usually about 20). Hibernation saves a copy of whatever your computer was doing so that when you come back to it and restart it can be right where you left off.
Hibernation (AKA Sleep) states are not appropriate for desktop computers. They are necessary for notebooks (when running on battery not house current) to preserve battery charge.
Some computers sometimes have a problem returning from hibernation and will need to be restarted in the normal way. In Windows 7, you can bring a system back from hibernation by clicking once on the primary mouse button. If you start to experience problems with your computer returning from hibernation, I recommend you stop using that feature and do a normal shut down.
Shutting your computer down. Windows does a lot of important things when it shuts down. If you do not allow it to do this, it may corrupt your system such that it can no longer be started. Always click on the bottom left button and choose shut down.
The button on the front of your system is an ON button. It is not an off button unless Windows does not permit a proper shut down. In that unusual case, you will need to hold the ON button in for several seconds until you hear it become silent. Beware of the fact that each time you shut down using the ON button, you risk corrupting your computer such that it will not restart.