A horses eye is kind of like trifocals and when their head is high they can see in the far distance and when it is low they can see thing up real close BUT CAN NOT SEE THINGS IN THE DISTANCE.
Folks like a horse with a low head set but buy horses with a high one and try to make it low.
Look at how your horse moves out in the field when he is just walking around and that is really how he wants to travel.
You can change it a little but too many put a horse into a frame that is for their own idea.
A lot of the frame is just style.