The overworld's primarily there to set the stage for the MSQ and other quests, as well as gathering. There's also the sightseeing log, taking screenshots if you're into that, FATEs if you're into that, Hunts if you're into that, and general derping around in. Every once in a while I like to hop on my Chocobo and run (not fly) around different zones just chilling and taking in the atmosphere.

Your sentiment's not wrong; once you reach a certain point in the game, if you're not into overworld activities then you don't need to leave a city and it turns into an "MM-lobby-chatting-and-people-watching-while-waiting-for-queues game". From what I've gathered from Live Letters is this design philosophy is intentional given the relaxed nature of the game.