The term 'must' has a few meanings: 1: highly recommended; 2: the quality of smelling or tasting old or stale or mouldy; 3: grape juice before or during fermentation; 4: a necessary or essential thing
because during the injection molding process, the injection mold temperature will get higher and higher.So need cooling system to keep it suitable temperature.
Nothing really tells people what they must and mustn't do, humans just know the right thing to do. Like you wouldn't need someone to tell you not to jump off of a cliff-we already know not to.