DJs are doing a lot more than just playing music before and after your event. During the event, a good DJ will keep everything on track and make it run smoothy from start to finish.
Weddingwire.com has a good article on exactly this topic, but to summarize:
Time - covers pre-event meetings, preparation, load-in, load-out, and the event itself.
Expertise - the know-how to handle special events, especially weddings
Equipment - attractive, high-quality gear that complements your special day
Music - standards, hits, and special requests, in high fidelity with proper licenses to play anywhere
Operations - rent, utilities, storage, transportation, presentation, wardrobe, insurance
Each of these topics has an expanded article that you should read if you are interested.
It's a little "inside baseball", but should make you more than comfortable with our qualifications and more than justify the cost of a proper special events DJ.