I understand your confusion. This range of movement exercise is the polar opposite of what we do in riding, these muscles get weak because they are not in contraction. Sitting on a horse, our hips are flexed (bent) and rotated to the outside - that is external rotation. The femurs are also spread apart, and the muscles we use from our hip to our ankles in adduction (closing our legs onto our horse) are far more active than abduction, which is taking our legs off the body of the horse (and hard to do as we're so tight).
That chair thing is only a means to check your weakness/strength for internal rotation of your hip (think femur). I am using it as a balancing exercise and to strengthen those weak muscles. I am too strong in adduction,but abduction is weak. The range for normal should be 45-50 degrees off the perpendicular.
My hips have gotten tighter since I've moved from riding 4 days a week to 6/7 in July. Location seems to be around the hip itself and sometimes the butt - although I suspect THAT is disc-y. I ride english- flat, some jumps, some dressage. I don't ride hours and hours, really only 1 to 1 1/2 but in my past I have run, played tennis, taught aerobics classes, dance, weight trained, etc. I think some of it is wear and tear. Plus I am 57.
Another exercise that is useful for the entire side of the hip/butt is wall squats using two balls (like those kids play balls about 6-9 inches). Place one ball between your knees. Stand sideways. Position another ball between the wall and just above the knee. Squeeze the ball between the knees, while pushing the other ball against the wall. Go into a little squat position and hold the contraction. Keep the chest lifted, abdominals engaged. As you improve your strength you can do small squats, holds, mix it up. The entire glute med, it band, tfl and the deep hip muscles will contribute to your stability. I do 2 or 3 sets of 10 reps. Argh. Plus as much stretching as I can stand. Maybe I should take up skating - another way to balance my muscles!!