Mentoring moves a young person beyond the familiarity of the adults in his personal life and provides a broader expanse of role models. The expertise is out there in the world. Good teachers know a great deal about human development and a great deal about their own subject area specialty. But they can't be expected to know about all the interests their students may have. The expertise of a mentor in music, engineering, banking, cooking, medicine, or whatever else a young person is interested in, is valuable to both the student and the teacher.
Mentors are information resources who can offer so much more than a book can.
The guidance is invaluable. There's a level of comfort in this kind of guidance that makes it possible to learn through both accomplishments and mistakes. For teenagers, it can be an especially important haven during the growth spurts of adolescence.