>If there's a famine or a meat shortage, wouldn't people turn to other foods on their own? So it seemed unnecessary and superfluous.

No, what will happen is some people will get meat for the week in their shop, and some will get none.

Imagine meat is like toilet roll, one day a family turns up late to get their food from the store and there is no meat left for them.

It's better to avoid situations like this by trying to convince households to eat more alternatives, than to leave some households with no meat at all or let meat queues start etc.