A disc jockey can do a lot of things at different events. Provide sound, wireless microphones, even video projectors. Of course music is essential, but it is not always the primary reason to hire a DJ.
A lot of work I do in the Dallas / Fort Worth area has to do with health and fitness clubs. I will often provide the right athletic wireless microphones for instructors of Spin Classes, Zumba, or Yoga. Sometimes the events are outside or poolside in order to give clients something interesting in their workout environment.
More and more, certain health clubs are becoming like mini-country clubs (sans the golf) with restaurants, social events, child care and other things all incorporated into a lifestyle or community event.
One such example is The Mat Yoga Studio in Richardson, Texas. Check out this video to learn more about their successful Karma Yoga Happy Hour.