[scott00]

Optimality's really never necessary. But it's a good idea sometimes. The two main ways of considering optimality are maximizing revenue and "efficiency". Efficiency is a technical term roughly similar to fairness and the overall good: it tries to measure to what extent auctioned goods go to the participants who most value them.

So going for an optimal auction can be a good idea if you're trying to make the most money possible (many if not most private sector auctions), or to distribute items in an equitable way (lots of government auctions fit this category).

In this particular case, airlines have multiple goals. They want to solve the overbooking for a small amount of money, they want to minimize the time their staff spends dealing with it, and they want to keep their customers happy.

An auction's a good fit for discovering how to keep their customers happy for the least amount of money in an automated way. As to which auction... I don't think theory helps very much here. As I said, theory doesn't work well except in high stakes auctions with sophisticated participants. So you need a behavioral model of your consumers (ie, an understanding of the ways in which they behave irrationally) to figure out a good choice in this case.