What a loose base costs you later
Concrete is strong in compression and weak in bending. Set a slab on ground that has not been compacted, and any spot that settles leaves the slab spanning a gap it was never built to span on its own. Eventually that spot cracks and a corner drops. Most of the garage and shop floors that come back with problems are not concrete problems at all. They are base problems that showed up months or years later.
Red clay makes this worse than it needs to be. Clay holds water and swells, and it moves through freeze and thaw. Ground that looked firm on a dry day in October can be a different story after a wet spring. Compacting the stone base is what keeps that movement from reaching the slab.
Why it happens in layers, not one pass
Stone does not compact well in one thick dump with a single pass over the top. It gets spread in lifts, worked with the compactor, then built up again, because a compactor only reaches down a certain depth before the force stops doing anything useful. Skip the layers and you get a base that looks compacted on the surface and stays loose underneath, which defeats the entire point of putting stone down in the first place.
The same logic applies to the subgrade itself before any stone goes on it. Soft spots get compacted, or where they will not firm up, dug out and replaced with material that will.
Where compaction happens more than once on a job
It is not a single step. The native subgrade gets compacted once the topsoil and soft dirt are stripped off it and the grade is cut. Then, once stone goes down on top of that subgrade, each lift of stone gets compacted again before the next lift goes on. Skipping either step and hoping the other one covers for it is how a base ends up looking finished on top while staying loose where it counts.
The last check on that work, before forms and steel go in, is walking the compacted base or running equipment over it and watching how it responds. A spot that flexes or pumps water under load has not been compacted enough, no matter how solid it looks.