I wouldn't say it really helps a horse..but to be fair, I don't think a fancy bit will help a horse do anything regardless.
I barrel race in a D ring snaffle at home and at events quite frequently. Honestly, the only reason I use one is because my horse likes it. Mine has rollers in the middle which he quite enjoys.
It does have a "Pay attention" to me aspect which I think can be good...however it seems that gets abused more than anything else. It's a bit that you need to understand (As are all gag and leverage bits in my opinion) and like always, the riders hands really determine the severity of the bit.