I think the latest on it is that horses do it to alleviate indigestion being brought on by being kept in & fed lots of hard food. Apparently it responds well to antacids, eg Settlex.
Some people think that wind suckers are more colic prone & harder to keep weight on.
I'm not sure if I'd buy one because I'd always be worried that they were in pain.
Mae windsucks and it doesnt put her off her food, she seemed to do it when bored in the stable, never does it in the field. She certainly hasnt lost condition, and it hasnt affected her performance, she just hates being stabled!!
I put a collar on her which has stopped her doing it, she only wears it in the stable. She only started it after i bought her and previous to that she was on oats and cool mix (not my choice of feed). For all the hard feed she gets 1/4 scoop of cool mix and 1/2 chaff, most of her diet is haylage.
We give ours garlic to help with digesting their food aswell.