I always just used it to confirm your last action on a POST —> GET sequence. Eg confirming that your save went through/rejected (the error itself embedded & persisted in the actual page). Or especially if saving doesn’t trigger a refresh so success would be otherwise silent (and thus indistinguishable from failing to click).

You could have the button do some fancy transformation into a save button but I prefer the core page being relatively static (and I really don’t like buttons having state).

It’s the only reasonable scenario for toasts that I can think of though.