I would argue that the point can be anything. As others have said, creativity is key.
Personally, I enjoy role playing. In real life, I am neither a knight nor a princess nor a demon nor any of the other characters I can play in SL. I can write those characters in the form of fiction (and I do), but I enjoy the element of surprise inherent in waiting for and reacting to others' posts. I also enjoy building. I used to own a clothing shop on an alt account. Finally, I enjoy seeing all of the things that other users have created. Sometimes, I even buy them. Second Life is an open ended experience. You can be, do, see, or create anything you can imagine. Ultimately, then, creativity is the point.