The problem JIT is solving is that raw materials inventory ties up capital (it costs money), and having warehouses of materials is money sitting that could be used for different purposes (capital expenditures, salaries, dividends, etc). Then, if for some reason the raw materials aren't worthwhile anymore (spoilage, tech change, etc), you've wasted money on something that needs to be disposed of. In theory, the goal would be to have no inventory at all.

JIT is a firm level decision, not a society level. It's up to each firm to decide how much of something they are buying.