Source, in full
Widely attributed response to a questioner doubting the usefulness of hot air balloons. See Seymor L. Chapin, "A Legendary Bon Mot?: Franklin's 'What is the Good of a Newborn Baby?'", Proceedings of the American Philosophical Society, 129:3 (September 1985), pp. 278–290. Chapin argues (pp. 286–287) that the "evidence overwhelmingly suggests that he said something rather different" and that the attributed quotation is "a probably much older adage".