You don't per say NEED one but it is always extremely useful to buy one. If you live in an area where power failure is the norm then I would say go ahead and get one.
You can never be too cautious when dealing with delicate equipment such as computer hardware.
You really only NEED a UPS though in a network server situation where the UPS will give the server time to finish anything in it's cache, clear it, and then safely turn off. Otherwise there will be data issues.
Because UPS equipment can be expensive, I would say just go with a good surge protector. As I said you only need a UPS in a network server environment.