Rappelling single strand is just like it sounds; you rappel down a single strand of rope. There are lots of situations where you might want to rappel single strand.
Some people are more comfortable rappelling single strand, especially light people. For rappels into deep or turbulent water, you can set the length of the rope so that the rappellers rappel off the end of the rope and don't risk losing their rappel devices in the water or having their rope get tangled up in the current.
Heavy people could rappel too quickly and lose control, especially on the tail end of long rappels.