If you are new to the SecondLife virtual environment then this video by San Jose State University will give you a good idea of the possibilities for educators in Second Life. The educational use of virtual learning environments can be as formal or informal as one would want. There are whole campuses built that are exact replicas of the University buildings in real life, complete with lecture theatres, seminar rooms and stocked libraries. There are also more inovative teaching environments that go beyond the existing constraints or notions of what teaching might be.
The whole point is to improve the student learning experience. To do what one cannot in a regular teaching setting or to improve on existing methods.