I've never useda wrap on my arrows. What are the reasons for these? Is it just because they look nice or is there a practical advantage? What is the downside, if any?

They add a little weight to your arrow (plus or negative, depending on what you want), any fletching glue works (typically better adhesion than to bare shaft with less fuss/prep), if you use bright or white color it makes it easier to find your arrow or trace it in flight to see where you hit. Also, its easier to remove the wrap to re-fletch without scraping/sanding glue all night. Downside, if you need to replace one vane, all of them need to be replaced.

he basically summed it all up

Yepper, do the same with white, it is easier to prepare, and that is the only draw back, replacing them all. I do about 6 and as they go i put them aside and then do several at the same time, and keep an exact set of six as back up for hunting!

I like arrow wraps as they customise your arrow and make them easier to see in flight and find after the shot.Fletchings adhere better to wraps also.I have never noticed that they shoot differently because of the minute weight added. I make my own instead of buying them. I go to a vinyl letter sign maker in town and they give me scrap rolls of colors I desire and then I cut them on a blade drawn paper cutter. 1" X 6". Very simple and beats buying them at $1.00 or more from the store.