Does the arch have a function while it is being built in your analogy ?
The arch is a useless pile of rocks until the keystone comes in. It can't even support itself.
But, the scaffold can both support the incomplete arch
and allow for travel across the canyon, at the same time. The purpose of the scaffold is to do both. That's how an incomplete arch can "evolve" step-by-step; on top of something that provides the same function, only not as well. (Scaffolds aren't as strong as stone arch bridges.)