Turning your computer on and off a lot will shorten its life span. This has much to do with heat. Electronics do not like to get hot (turned on) and then cold (turned off) repeatedly. Heat causes expansion of materials and cold causes the contraction of materials. Thousands of this on-and-off eventually will cause electronics to slowly break down and eventually fail. Hot cold hot cold eventually equals a failure.
TIP: If you rarely use your computer, turn it on and off as needed. If you use your computer frequently, leave it on all the time unless you plan to be away from it for a long period of time.
Heat is one of the main causes for laptop or desktop computer failure. Heat can slowly cripple and then kill your computer. Computers, like most electronics, like to be brought up to their preferred operating temperature and left there. A stable temperature maximizes the operation and life of the machine.