Most of the time when there is a new firmware update, you can't sign into the PSN until you update your PS3 with a required system software update. But there are these rare few times when updates aren't mandatory, and PS3 owners might miss 'em. Firmware 3.73 is one of the latter cases.
It's not mandatory, because it doesn't do much. It provides "software stability" on certain PS3 titles. If you're experiencing any game crashing, or other bugs, you might wanna update. Otherwise, this is a firmware update you can pass on.