In some scenarios, it is desirable to have the rope stop at just the right height. Then when you reach the end of rope, you rappel off the end and don't have to spend time unclipping. Scenarios where it could be beneficial to set the rope length include:
There are several different ways to set rope length (refer to the table on rigging a retrievable rope), but all of them involve rappelling single strand.
When setting the rope length, it is critical that you don't set the rope too short, otherwise the first rappeller could fall and injure himself. If you can't see the bottom clearly and have no way of knowing if you set the rope length correctly, you can lower the first person down and then set your rigging at that length.