Second Life is the leading 3D virtual world. It's a space where you can be whoever you'd like, build and sell whatever you can imagine, and have fun with others from all over the globe while you explore unique virtual environments, listen to live music performances, play games, shop in the world's largest user-generated virtual goods economy, and much more.
Quite simple, It's the only one of its kind so far and it's an astonishing achievement that it still exists and being in development anyway. You can't compare it today as it was 9 years ago. So, there are still technical flaws they keep working on, like the lag many complain about. It's reasonable faster today. Just keep in mind that this platform runs at 100% in real time. That goes for everything you do or see and they manage to keep the entire grid of roughly 26k regions up and running for 24/7. So, what's the mission in SL anyway? Well, that's like asking: how far can I take my imagination. You could spend a life time to figure out all its creative tools, collaborate or improve your sense for fashion. SL's avatar fashion is the most sophisticated of any virtual worlds/sandbox games and you really don't need to spend a dime for quality stuff. Or you can meet some of the most interesting people from around the globe. Finding people who are not socially inept can be one difficult task, yes, but that goes for the entire Internet these days. The average intellect is far away from YouTube comments after all. There are hundreds of subcultures and mini games around. It takes some time to get comfy with the mechanics but that can be a lot of fun with some patience. Immersing can help to learn about unconscious aspects of yourself, boost your self-confidence in real life, or improve social skills. They don't say for no reason that SL is a paradise for Asperger's. Of course, such a platform contains psychologically risks but that just proves its potential. Just don't take it too serious and have fun.