So you can make really cool looking arrows.. Most people add them for visibility and customization. There are all kinds of wraps out there, and some of them are really neat. For me, they look cool and I can see the arrow in flight much better. The down side: If you need to replace a fletch, you most likely will have to remove the entire wrap and redo the arrow. Also, they do add some weight to the arrow, but not much. Mine added only 10 gr. I can live with that.
They also allow better adhesion on carbon arrows. The fletching will stick much, much better with less glue.