These instantaneous, exceedingly high voltages are caused by lightning strikes or power resumption after a blackout. Brownouts , also known as sags. These short periods of low voltage are caused by the start-up of heavy equipment or the reduction of voltage on utilities to cope with extended periods of high power demand. Brown outs or sags can crash your computer and damage electrical motors and other equipment. Noise , or dirty power.
Dirty power can cause errors in computer files and programs.
You may think that surges, spikes, sags, and dirty power are so rare that they aren't worth worrying about. But according to a study done by IBM, a typical computer has to deal with more than 120 power problems a month. Computer power supplies will handle many of these problems, but others could cause keyboard and hardware malfunctions. At any rate, power protection is cheap compared to the cost of electronic equipment. So it's worth the effort to take steps to protect that equipment.