The main benefit of a rehearsal is for the parents and bridal party to know how and when to walk in and walk out for the processional and recessional.  Everything in between is led by the officiant, who is the professional you hire to make the whole ceremony move smoothly even if there are a few bumps.  With a good officiant your guests will either never know about any "mistakes" or they will share an endearing laugh with you that makes your ceremony even more unique and memorable.

I always provide couples with a complete typed copy of the ceremony ahead of time for review. During the rehearsal, giving this to the wedding or site coordinator.  They are most likely going to be lining up the wedding party and sending them off as the music cues on the big day.  The officiant will already be at the front as the attendants begin to process - the coordinator will be there by her side through the moment the bride heads down the aisle.

The hour or so before the ceremony is the time when the professionals involved in the ceremony will coordinate with each other, and go over cues and final changes in lineup. They surely want everything to look as good as you do when the ceremony begins.

During the rehearsal, I will be trying to tell the bridal party their part to play, and directing everyone on what will happen.  Music will just be a distraction and one more thing for me to have to talk over.  Professional musicians and DJ's will have no problem following my lead during the ceremony.  I don't feel there is any benefit to having them set up their equipment while I show your group how the ceremony will go.

Especially if you have a large wedding party, rehearsals can bring some peace of mind to all involved and are a good idea. However, I think couples should not count on leaving a rehearsal feeling like they know exactly what to do every moment of the ceremony. Putting that weight on yourself instead of letting your officiant, coordinator and DJ/Musician work together like the professionals they are can lead to the bride and groom going through the motions rather than enjoying their one-and-only wedding ceremony. The job of the bride and groom (and parents) during the ceremony is focusing on the joy and sacredness of one of the most precious and important moments of their lives!