Anyone tell me what the advantage of using a curb strap on a snaffle would be?

it stops the bit turning insideout when the reins, are brought forward over the head.specially useful with split reins.
Also, whilst using an open rein on a young horse, the pressure is carried through to the opposite side, giving a slight push as well, as a pull.

If using a myler type snaffle with hooks,(Cheekpieces in the slot at the top of the ring) the curb strap can come into use, on the curb groove, encourageing the horse to flex at the poll.

also you want a leather curb strap on a snaffle bit not a metal one. see WES and AQHA rule books for pics.
rosie

ps i think western scene has some on their website.