There really isn't a point, curb straps are designed to keep the curb from tilting "up" too much, and because a snaffle can't really tilt, well.. You get the point.
I can see how if it was moderately tightened it could keep the bit rings from jingling on an oversensitive horse, and act like a flash on an english snaffle bridle, but I don't really know how effective it would be...
I've never heard it called a curb strap. Here we call it a lip strap, and it's different to a curb strap/chain... But yeah, loads of stockhorse people use them. It's helps prevent the bit from pulling through the horses mouth suring tight turns or high adrenalin work, especially used in campdrafting.
I always use a chin strap with my snaffle. It prevents the bit from sliding through the mouth. I don't put it on tight, either. I don't want it against the horse, just tight enough where it prevents the bit from sliding.
I use it on my snaffles, including my french link. I clip the reins on between the bit and the curb strap. Initially I did it to prevent the bit from pulling through he mouth on a bit of a stinker horse I had, but I have kept it up because especially with the reins in between it does help communicate better. Just another point of contact.