What is a ups used for in computers?

A UPS (Uninterruptible Power Supply) allows a computer to keep operating in the event that main electrical power is loss. This may allow just enough time to save any work and shut down safely, or it may provide enough power to keep operating for a long period of time until power can be restored.

A UPS device will also contain a surge suppression device to protect against power surges, but you can get these without purchasing a UPS.