OK here goes: There are two highscore lists in the Metaverse. Game characters and Empires. Your game character will compete in the characters list anyway but if you join an empire your score gets also added to the empire's score and you will compete with your team mates in that highscore list, too. So basically you work together with other people to bring your empire to the top. Besides: It's also a lot about having fun and interacting with other players (as well as bashing people from other empires). It's a second source of motivation, I guess. And you can role play a bit if you want - that's why I founded a BSG empire, because I love that show and wanted to recreate a bit of its excitement...
BTW, welcome to GalCiv. You will love it, I'm sure. Maybe you'll also discover that the Metaverse way of doing it can be as exciting (or even more so) than "real" multiplayer.
