We tend to think that our weight may not have much of an impact on the horse, but it does. How would you feel it you were carrying a bowling ball around? To be more accurate, what if that bowling ball was bouncing around? It would be significant and straining on the muscles. The horse’s muscles react in the same way if the rider is too bouncy or unbalanced it causes strain on the horse’s back and muscles. Correct posting can help relieve this for the horse.
The second benefit is for the rider. The rider experiences less muscle strain and fatigue when posting as opposed to a sitting trot.
For these reasons it is important to learn to post well.