I think EU is pretty logical extension if you have both EEA (free trade) and Schengen (free movement of people). If USA was just a collection of independent states with free trade agreement and free movement of people, we would live in a very different world.

Benefits is that there are a lot to negotiate internally besides free trade agreements and it gives negotiation power in the global arena, now that there are at least 3 superpowers (USA, China, Russia to the lesser extent).

(P.S. Didn't downvote)