Deposits guarantee the services of a professional for your event date. The deposit shows the event professional that you are serious about using their services so they will be glad to turn down other work for the same day. If you come across a DJ that does not require a deposit then at least get a contract for some type of verification on your end. Otherwise you will roll the dice if the DJ will even show up, especially if someone else wants to hire him or her at a higher price.
Thankfully with Great Rate DJs our deposit amounts are small, usually $95 unlike other companies that typically charge at least 50% of the total.