A minhag. And one that bespeaks affluence.
Let me tell you about another minhag that stems from a time where food was never a disposable commodity. On the second seder night (chutz la’aretz) someone I know has the minhag not to finish the entire piece of afikoman. Instead, they save a piece “for next year” in case they are unable to make matzos for some reason. Clearly there is no halacha prohibiting this.