It would be hard to run an oil company if gasoline had to be produced at the refinery at exactly the rate the cars were burning it. It isn’t feasible to harvest a forest at the precise rate at which the trees are growing. Gasoline in storage tanks and wood in the forest are both stocks that permit life to proceed with some certainty, continuity, and predictability, even though flows vary in the short term.
Stocks allow inflows and outflows to be decoupled and to be independent and temporarily out of balance with each other.
