-Fighter protection: Standardized fighter licensing and drug/medical suspensions from state to state are crucial for the protection of fighters.
Generally, athletic companies are strongly against the organization of their employees; one needs only to look at the various union problems sports leagues have had (and that boxing has never gotten their act together in that regard). But this doesn't have to be full unionization of fighters: just a guarantee that guys won't injure themselves hopping state lines to make a buck. It would protect fighters from themselves; and if it leads to an insurance program down the line, all the better.
For state governing organizations, the return for giving up power to a national board is that much of the logistical legwork tracking fighters could be taken out of their hands, meaning less work. What bureaucracy doesn't love having less to do? Meanwhile, they could still impose state-specific requirements above and beyond what a USMMA would require.
-Lobbying power: Locally and globally, a USMMA organization could bring combined efforts to bear in lobbying officials. The two major areas where this could be a boon for mixed martial arts are in legalization in the US and in international profile. Want MMA in New York? What about in the Olympics? A unified front by a United States MMA commission - with input from all the major and regional US promotions - adds heft to MMA's sanctioning efforts.
-Staying power: Every soccer field in the fall is filled with packs of roving children, usually inches apart from each other, flailing away at the ball and having a blast doing it. While MMA likely won't have the same sort of massive youth pull, soccer provides a good model for getting kids involved and building a sport's community from the ground up. I firmly believe the MLS would be dead already, instead of doing well for itself, if it wasn't for the emphasis on getting kids into soccer in America.
Martial arts are popular with youth already for a variety of reasons: self-defense, empowerment, and the sense of achievement kids get moving through the ranks at their local karate or tae kwan do dojo. A USMMA organization could help to bring youth along into MMA, perhaps with an early focus on jiujitsu and grappling before moving into bag striking as the kids get older so that by high school they're ready for contact. (The local high school wrestling programs would likely not complain they were getting more talented grapplers, either.)