The butterfly bit, although very short shanked, allows for different rein positions (like a ridden Kimberwicke or Pelham bit or a Liverpool driving bit the latter two which has a much longer shank) and a curb chain (needs very small headstall and the type chain with hooks). It falls into the category of shanked bit (curb), working off of leverage and curb chain but having a "broken" mouthpiece.
I liked the eggbutt for riding as for some reason they didn't have a tendency to pull thru the mouth like a ring snaffle sometimes did. Never really figured out why - and it still will if you pull enough or if pony resists turning.
Hmmm, I like that butterfly bit... Don't really have any right now that "need" that type yet, but I love collecting and I'd rather have it on hand when I'm ready to try it out...