While living in trees, being able to grasp onto branches with your toes is extremely useful. See [how the foot of a chimp looks like](https://www.sciencephoto.com/media/535109/view/chimpanzee-foot), for example.
After we moved down from the trees and stared walking upright, the need for grip disappeared, but the toes adapted to help us with walking. It’s easier to balance when you have toes to work with, and you can easily tell how you start engaging your toes more as you go from walking to running.
(Incidentally, your pinky toe is almost irrelevant for that purpose since your other toes can do its work, so it’s entirely possible that evolution ends up giving us four foes in each foot down the line.)
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