Yes,
socializing. I've had some pleasant conversations with people I'd never met before, sitting in a nearby park where somebody had put up some lures. And this pattern has repeated a number of times for me personally and I've heard many similar stories from others. Playing Ingress, which is Niantic's predecessor to Pokémon Go (in some ways, PoGo is a mere re-skin of Ingress), I met a whole bunch of people, some of whom are now very close friends. Socializing doesn't
have to only involve bars, sporting events, or alcohol, you know.
(Actually, given that Ingress is a fairly competitive, strategic, and very physical game, one school of thought holds that it is
a new kind of sport for nerds.)
Despite the crazy success,
this game, PoGo is still in its very early stages - judging by what Niantic has done with Ingress over time, PoGo will get more involving, more team-based, and likely will require players to work together to accomplish larger goals (leading to periodic massive organized events - Ingress already has these). In Ingress this has led to large social communities growing organically all over the world.