Why is it that computers need to reboot?

I assume it’s all kinds of computers, but the other day I got a message from Windows 7 telling me that it had been a week since the computer actually shut down and that it “needs to reboot to normalize the system.”

I’ve had this before, sometimes I don’t realize for how long I actually don’t turn off my laptop and just close the lid when I’m done. But I often wondered why a computer needs to “normalize” and what that actually means. Is it like the computer equivalent to how a person should need sleep? What would happen if you just left it on forever (or, well, just a super long time)?

Sidenote, but it made me think of The Matrix Reloaded. Ha.