After two lightning strikes knocked out power at our house, followed by several power breaker failures, we thought it was worthwhile investing in a UPS. We use RAID to protect against drive failures and the cost of protecting against another power failure seemed like a good investment. The idea behind a UPS is that you seamlessly switch power to internal batteries when mains power fails, but also signal that power has been lost and how much battery time remains. For example, you may tell a server to power down gracefully when there is 10 minutes of UPS battery time remaining. You also gain some protection from surges and spikes, which may protect against damage during lightning strikes. Link