The reason is simple... In order to go through the Registered process, you MUST show you have a legitimate TM to the government. You give them proof of usage and timelines and every other hoop they might throw at you. If they accept your registration, you have proof backed by the government that you have a TM. So if a time arises that you need to prove your TM, all you need is to show your government registration.
DNQuest nailed it. You don't *have* to register your trademark in order to have an enforceable right. However, in the US, registration is presumptive evidence of ownership and validity of the mark. Also, registration provides a presumption of "notice" on the part of an infringer, which can provide for an award of monetary damages as opposed to merely an order forcing the infringer to stop.