Imagine three or four of your best friends standing around arguing over what song should be played next, while all of your guests on the dance floor leave and go back to their seats.

Who is going to coordinate and carry out your grand introduction?

Who is going to check to make sure vendors are ready for each event as they occur?

Who is going to announce your cake cutting and get the crowd involved?

Who is going to properly draw attention to your first dance?

Who is going to coordinate all the people participating in a bouquet and garter toss?

A professional DJ, will assist with planning, coordinate with vendors day of, act as master of ceremonies, provide a diverse selection of music encompassing all ages, use experience to choose proper music at the right time, and will be your representative to your guests at your reception. 
The professional DJ is normally an expert in use of lighting (and provides the lighting) and in use of sound equipment and will take care of all those aspects at your reception. 
If providing these services on your own, the person renting sound gear, must:  select, pick up, deliver, move, set up, test, and operate what is hopefully professional level sound gear.  At the end of the event they have to tear this down and return it to the facility where it was rented. 
This does not include a selection of music or any experience as a DJ in reference to programming music, or as a master of ceremonies coordinating an event properly. 

The American DJ Association created the video shown below to help prospective brides and grooms to understand what they're getting into when they do an iPod wedding.  Please take a few moments to watch this video, the 5 min.  could save you the success of your event!
 

You may also want to read this article from a blog in Pittsburg, PA:
It is the 3rd article down on the page