This question was asked of a friend of mine a while back. The person who asked could not understand why anyone other than military, law enforcement or security personnel would want to take martial arts unless their goal was to harm others.
It would be tempting to task this person with researching Aikido and Zen and Budo and Bushido and the philosophy of the arts to provide answers, but that's may be unrealistic.
There are a myriad of reasons to study martial arts, but the two that most quickly come to my mind as an answer to this question (other than job related) are:
1. To protect your family and loved ones.
2. To avoid confrontation.
Number one is pretty easy to understand, it's number two that's tricky.
J.C. has an interesting quote over at Bujutsu: The Path
So, removing any work related reasons, how would you answer that question if someone asked you?