Different reasons, one reason is so the lamp can cool down after the projector is shut off in the event of a power outage (though many would argue that a lamp does not lose longevity unless you turn it back on within 1 hour after a power outage). Hence, let it cool before firing it back up and it should be fine. That said, voltage anomalies besides just surges can occur (though not as common today as it used to be), and a voltage regulated UPS will protect not only against surges but also against voltage dips. The power it produces is cleaner than going straight to your outlet. Some EE's would say it doesn't matter, but better safe than sorry, and I think it does help in some cases.
I recommend Cyberpower Pure-sine voltage regulated UPS, you can get them under $120 or much less depending on how many VA you get.