Pronationis natural. In inward rolling of the ankle. Pronation is really at the ankle. It allows for the foot to rotate inward as the tibia moves forward. It allows the foot to access the 4th arch along with providing short ground contact time when moving, If you are skiing, skating it allows for moment of the skis along with tensegrity between the feet and and pelvis.
Now some see pronation as the collapsing of the arch. At insoles by Barrunning we don't see pronation as that the arches of the feet should collapse. They should hold firm and solid to support you moving over the top of them.