Someone please correct me, but I think its to take the bulk of the buckles from underneath your leg for a closer contact with your leg against the horses side.
Ahhh thanks, I can see the reasoning behind it though not in practical terms, having both a dressage and show saddle [used on the same horse], dont notice any difference in the way of going. I ask as I am thinking of changing the girth strapsto neaten the look and using the dressage saddle for showing as it is more comfy!