Today I was asked a very deep question by a newbie: "What is the purpose of Second Life?" Lots of newcomers ask this, although it's usually worded so as to be a lot more limited: "How do you play this game?" Both are really asking the same question, although the first query at least shows that the person asking it isn't wearing the philosophical blinders of thinking of SL as a game.
My first response is another question: "Well, what is the meaning of life?" Second Life is a virtual world, full of real people and their works. In this, it is like the real world. The purpose of life, whether First or Second, is whatever the individual decides it is.
What's YOUR goal in life? To get a good job? To become famous? To do something better than anyone else? To become rich? To find true love? To teach? To explore? Do you like to tinker with things and try to make them better? Do you want to live in luxury? You can do all these things in Second Life, just as you can in Real Life. And in the virtual world, the risks and consequences of failure are much less...so you can take chances that you never would in real life.
Or perhaps your goals are less laudable. Maybe you are out to take what you can, and to hell with anyone else. Maybe you are bitter, and have decided to try and make everyone around you as unhappy as you are. The consequences of getting caught are less in a virtual world, and so Second Life can bring out the darker side of people every bit as much as it gives others unequaled chances to succeed.
It's not a "purpose," but it is a characteristic of the virtual world that it acts as a magnifier. The good become better, and the bad become worse.
Linden Lab gave us a world, and the tools to shape it. They did NOT give us a purpose. What some people see as LL's failure to provide a goal for the "game" may in fact be the very best thing they could have done. Whether it is or is not, it is in any case a grand social experiment.
But back to our original question. My dear young noob: Second Life has no purpose! Shock! Horror! Confusion! You must find your own. That can be pretty scary, I know...but you know what? That's called "freedom."