An uninteruptible power supply or UPS for short, provides vital protection to computer systems, servers, network equipment, DVR, NVR and the like. When hydro fails from the power company, these systems shut down abruptly and this can cause major failures caused by crashing hard drive heads, cached data that's lost, etc. The battery backup in the UPS allows those systems to stay up to avoid short power blips. In some cases they can also be connected to the main system so it can be shut down if power stays out for a longer period of time.
Every single computer, server, DVR and NVR should have a UPS battery back-up.