The stirrup bar is attached by the saddle manufacturer in a way so that the stirrup leather neither slides off while riding, or gets stuck in the event of a fall and the riders foot is stuck in the stirrup.
With the stirrup bar parallel with the ground, you can determine where the deepest point of your saddle actually is which is where you should be sitting.
If, when the saddle is placed on the horse, the stirrup bar is pointed downwards in the front, your saddle is likely too low in front and causing you to places excess pressure through the front of the saddle. Additionally, it may also cause the rider to sit against the pommel instead of centered in the seat.
If, when the saddle is placed on the horse and the stirrup bar is pointed up in the front, the saddle is too high in front and placing excess pressure along the rear. This can also cause the rider to feel like they’re falling back/falling behind the movement of their horse and having to “catch up” in the gaits.
The ideal weight distribution should be close to even along the entire panel.
Take a look at your saddle the next time you ride.