Volume is a variable
In paid channels, creative is the largest controllable lever left. Volume of tested creative predicts performance better than almost any targeting decision, and volume requires a system.
What a system contains
Fixed elements that carry recognition, variable elements that carry the test, and rules that make the difference obvious to anyone producing assets.
The governance part
Systems decay. Quarterly review, versioning and a named owner are the difference between a design system and a folder of old files.