1) Groups get a speaker for FREE
2) Get feedback on why people do not use the library
3) Helps you practice your presentation skills
4) Sign people up for library cards
5) Shatter stereotypes about the library (I had a group that was surprised that a guy could be a librarian)
I am sure there are other reasons as well, but these are some that come to mind personally. If you have other ideas, e-mail me!