Example: We schedule events which require someone somewhere go pay for room hire. Rooms have a maximum capacity, so to recoup the cost of hiring the room we then sell tickets to the event, but we don't want to oversell. If we could set the number of said upgrades available to the max capacity of the room that would be super handy. The upgrade we associate this with is membership to a group which has access to the event details - maps, schedules, and so on.
Second Example: Our users love the idea of collecting limited-edition trophies. If we could limit a new trophy to, say, 50 sales, then it's a first-come first-served to gain a pretty icon for their trophy cabinet