The post office sold the forever stamps to fund its operations, and they were guaranteed to be able to be used for a specific utility (mail a letter) for which the price would increase over time. So people could "invest" in the stamps now and be guaranteed a profit over time as the cost of mailing a letter increased. In fact, many people did purchase large quantities of forever stamps with the purpose of reselling them at higher prices later.
Wow that is a very insightful analogy.
Makes sense.