Another factor is earning elite status with an airline. This has benefits which often can't otherwise be bought for any (reasonable) amount of money. That's not enough to justify flying in the first place, since if one isn't going to fly the benefits of airline status are useless, but if one's other flying adds up to (say) 48,000 miles with American in the course of a calendar year, they justify a cheap December trip to reach 50,000. The exact milestones vary from airline to airline, and most airlines have more than one with more goodies as you move up the ladder, but the idea is the same.