To keep the horse from throwing their heads and getting distracted in the ring. It’s the same purpose of the standing martingales but running martingales are gentler
It’s good for sensitive horse because the rains can’t move around in every direction. All user’s of a close ”v” martingal have diffrent reasons, sensitive horse, stallion, fashion thing.
It sharpens te movements of the reins, because the breastplate is close to the chest incomparison to a regular martingale
The only purpose I know is that it is for stallions who sometimes bite in the open martingale straps. When using a closed martingale, they cannot get caught up in the straps with their mouth.