The cantle in the rear gives support to the rider’s back and buttocks. Too steep and it can bruise your back side or take too much room from the seat squeezing the rider forward and making it uncomfortable against the swells. Too shallow and it will give you no back support nor security in the case of an unexpected lunge forward.
A deep cantle has many advantages. It supports your lower back better over long rides. It holds the rider in place better which causes less “wobbling” of the rider and less unnecessary wiggling in the saddle on the horse, which can cause soreness to a horse’s back.
Most cantles today are about 3-1/2″ to 4″ high and have a gentle curve across the top. They do not hug too tightly.
A 4″ cantle is usually very good for the average rider. It gives sufficient back support and security for most situations.
Endurance Riders or range cowboys might like a 5″ or 6″ cantle for even more back support.
Height and slope are different. Don’t confuse them. High Cantle can still have a low, medium or steep slope.
Trail riders often like a 4-6″ height with medium slope.