If it goes up in smoke, you personally can't ever buy enough ammunition to matter. Who's gonna man all those guns?

This depends heavily on geolocation. Experiencing economic collapse in the Montana wildnerness would be very different from experiencing it in Washington DC.

I wasn't thinking of fighting off hordes or anything like that.

Ammunition can be used for barter, hunting, and self-defense. It can also be used as a tool to break locks, start fires, and send signals.