What benefits are there to university alumni mentoring undergraduate students? My faculty is starting a mentorship initiative where they match undergrad students with alumni from the same program. I was contacted by one of my old professors, and asked whether or not I could be a mentor. 

Now I have been in mentorship programs before; and I currently have a mentor myself, but I have always been the mentee, never the mentor. I have to say I never considered what the mentors have to gain in these programs. Is it mostly charity, in the sense that you selflessly help/guide the mentee, or are the intellectual/practical benefits to mentoring, particularly within academia?
What can a mentor gain from the mentor/mentee relationship?


Altruistic answers:


Passing on learned knowledge and lessons is a hallmark of a society. What you learned as a student, employee, employer, citizen, etc. is important so that once your brain and the brains of others in your generation are pushing up daisies, the knowledge isn't lost, and repeating mistakes can be avoided. Many people consider this a satisfying endeavor.
As a role model, you can demonstrate moral, ethical, compassionate, etc. strategies for using the education you got from the school. Knowing that you've led someone down an ethical path is a good feeling.
You can act as a sounding board for your mentee's questions about your field, and for the types of questions that don't routinely come up in class. There is a certain satisfaction in being able to answer questions based on your experience.
There might be new ideas that your mentee knows about your field that you can learn from. While you pass on your wisdom, you also have the opportunity to learn some of the new ideas that are being taught today.


Less altruistic answers:


Depending on your field, having been a mentor looks good on a resume or in an interview.
In your case, you'll gain points with your former professor, who might be more inclined to write you a good letter of recommendation some day.


Decidedly bad answers:


Steer the kid in the wrong direction, and that's one less competitor!
You might get your lab cleaned because "it builds character."
In academia, if your mentee has good ideas, you can steal them and publish the results yourself.