This post centers on a mobile game called Ingress. Like lots of mobile games, this is a great game to play while traveling around to waste some time. Unlike most other mobile games, Ingress is a game that you have to play while traveling and helps you discover the world.
There is a fairly elaborate sci-fi story and world behind the game, but ultimately it boils down to a world-wide game of capture the flag(s). The "flags" in this case are historical markers, public buildings, park signs, murals, and anything else that is publicly accessible and significant culturally or artistically (you can even submit suggestions!). Once you have found portals, you can attack them to remove the other team's control, claim them for your team, get items from them, and link them to other portals.
Like almost all games that you play online, the entire thing is almost completely pointless. You run around interacting with your phone to control imaginary "portals" and gain imaginary points. Everything is just made up.
Yet, thousands of people around the world play this game actively, to the point that they will travel hundreds and thousands of miles to participate in special events around the world as part of the game. There is obviously something about the game that hooks people.