Does it really need one? No. There are plenty of folks that have successful tanks using dechlorinated tap water.
BUT... in my opinion, if you're going to keep corals, you should have one. Consistancy is the key to success in this hobby, and without a consistant water supply that is also pure, you really don't know what you're putting in your tank. With a RO
unit, you have consistantly pure water that you're starting with, and YOU (and your salt manufacturer!) control what goes in it.