As you supposed, it is about the movement, walking through holes and such stuff. The capsule component doesnt fit these stances, as the height cannot be lower than the doubled radius (it is just a sphere then). However, when the character lies on the ground, the only possibility to set the height to such a small value as 15 cm for example, is also to set the radius of the capsule to 7,5 cm, which is very indiscussable I think.
I would prefer a flat box as collider for this stance.