Did you know that the hindquarter is wider than the front end and the hips are wider than the shoulders?
This is a dimension of the natural asymmetry and it's called the front-back ratio: the hindquarter is wider than the front end and the hips are wider than the shoulders.
This is easy to see from above:
So a horse has a bit of a 'torpedo' shape.
Now are you aware of the position of your horse's shoulders in relation to the hips when riding?
Most horses walk a little bit 'haunches-in' like, even when this 'exercise' is not asked for.
Because when the horse is not straightened along the wall of the arena, this effect is enhanced. When the horse steps too much to the inside with its inside hind leg, the hind leg cannot step under the center of mass.
This inhibits the development of the carrying capacity of the inside hind leg. The carrying capacity is what is important in order the carry the rider.
Another pitfall is that this causes the horse to lean with the outside shoulder against the wall.
As a result it's sometimes impossible to get the horse away from the track and to make a turn inwards.
Simply riding forward hoping that the horse will straighten itself out leads to stiff, "hard" horses that lose the flexibility in their back and as a result in their mouth.
This is why it is so important to do Straightness training!