Why do we have arched feet?

We used to think it was so we could walk faster, helping lift the next step.

In fact, the spring like arch in our feet stabilizes our ankle. It does allow us to run more efficiently, but without an arch we wouldn't be able to be full-time bipeds in the first place.

(SF writers designing aliens take note! You need some mechanism to stabilize the lower leg. And furry digitigrades don't work, alas).

If there's no recoil then you walk like a chimpanzee, and they aren't nearly as good at walking upright as we are.

Could this be useful? It could be because some people might have an injury or illness that stiffens up their arches, which then impacts the ability to walk and puts strain on other joints. We might be able to work out some therapy, or even special shoes, that help make the arch more flexible.