When this equipment is switched off, the extra voltage is dissipated through the power line. Computers and similar sensitive electronic devices are designed to receive power within a certain voltage range. Anything outside of expected peak and average voltage levels will stress delicate components and cause premature failure. Electrical noise is caused by many factors and phenomena, including lightning, load switching, generators, radio transmitters, and industrial equipment. It may be intermittent or chronic.
Noise introduces glitches and errors into executable programs and data files.
Below are some figures from APC's Web site, dealing with the cause of computer problems. The source for the information is Contingency Planning. As you can see, power problems are the main culprit. APC has a wide array of power protection products available. Simply put, a UPS is multiple outlet surge suppresser with a big battery.