There are many, many reasons to use a UPS, but using your example of DB transactions, look at it this way. Even if a transaction is committed, you lose power, and your RAID write cache (you are using battery-backed write cache, correct?) batteries aren't functioning properly, you're out of luck.
Additionally, power outages are notoriously hard on equipment. Replacing a $500 workstation is one thing, but it's much more painful (both financially and otherwise) to replace a server that costs an order of magnitude more (data on that server notwithstanding). Most good UPSes also are line-interactive and will offer much better protection from power surges and brownouts than you'd get with a cheap power strip.
Overall, it's just a very well-accepted best practice.
I do understand that it can be useful on servers when changing the server configuration, but that is done during a very limited time and it sounds like it's more worth to do good backups before the configuration.
What does using a UPS have to do with changing a server's configuration?