The software doesn't necessarily have to be installed, but as mentioned above, it is a good idea. A UPS has some batteries inside (usually normal car batteries), and these will only sustain the power for a certain time while the mains power is out. Once the batteries are drained your servers will just turn off ungracefully if there is no software applied. The main reason for UPS is to keep the servers running for a short time during power outages, and once the batteries are close to being drained, to shutdown the servers gracefully so no data is lost. If they are just turned off you can loose data or get filesystem corruption. And for that shutdown to occur, the UPS has to tell the server about this and for that the software is needed (as hinted by the posts above).
