A prototype exists to answer a question before the expensive work starts. Generated assets change which questions it can answer cheaply.
What grey boxes cannot tell you
- Whether the space reads at a glance.
- Whether an object is recognisable at gameplay distance.
- Whether the art direction holds together.
- Whether a player understands what is interactive.
All of those depend on things a grey box removes by definition. Teams used to defer them until art production, which is exactly when they become expensive to change.
What a populated prototype answers
Dropping approximately-right props into a level takes hours rather than weeks, and it makes readability testable early. If players cannot tell a door from a wall when both are grey, that is a level design problem. If they still cannot when the door looks like a door, that is a bigger one — and you now know before the art budget is spent.
The trap
A prototype that looks finished gets treated as finished. Stakeholders see populated levels and assume the art is done; schedules get compressed against a milestone that was never real.
Two habits help. Keep the generated pass visually distinct from the final art direction — deliberately uniform, deliberately untextured, or deliberately off-palette. And say plainly, every time it is shown, that this is a blockout wearing clothes.
Throwing it away
Assume you will. Prototype assets carry no budget discipline, no naming convention and no LODs. Some will survive to production because they turned out to be good enough; planning for that is optimism, and planning against it is cheap.