for using at 3-day events on temporary stabling, to stop wind/rain/other horses' bugs getting in, maybe? i've seen people using plastic sheeting and stuff, this looks like a posher version. could be wrong tho.
Seen things like it at Ponies UK. Someone had them on his stallions doors so they couldn't get wound up but the constant moving of horses around them. And partly he told me because muppets lead horses right under their noses and get s**ty when they take a chunk out of them.
I have one instead of a little roof on my stable door, it stops the rain coming in, or in hot weather, keeps the stable cool, they have ties on so you can roll them up like a blind, mines a mark todd one.