The problem with the more common types of breastplate ('hunting' type and breastgirth) is they have to be fitted loose to allow the horse to stretch in front without pulling the saddle forward. This means the saddle can slip back a handsbreadth, far too much in competition.
The idea of the 5-point is the elastication allows it to be fitted closer to the horse without restricting forward stretch so if the saddle should slip it can't go far enough back to be out of balance.
As far as I know that's the theory, anyway. And they do look pretty cool :)