Frequent-flier programs are becoming one of the most prized assets in aviation.

Loyalty programs are now indispensable to some airlines, generating healthy profits in good times and blunting losses in downturns. By one estimate, the programs are worth hundreds of billions of dollars globally, accounting at times for a large share of the value of the companies that run them.

The loyalty ecosystem is fueled in large part by credit card spending, particularly in the United States, where it is widespread and most major airlines work with banks to issue cards tied to frequent-flier programs. Three of the biggest U.S. carriers — American Airlines, Delta Air Lines and United Airlines — earn billions of dollars annually from cards and loyalty programs, revenue that has been vital to their success.

“A very large share of the profits are directly attributable to the loyalty program,” said Evert de Boer, managing director at On Point Loyalty, a consulting firm. “If you were to take away the loyalty programs, it would have a huge impact on the big U.S. carriers.”

The programs have reinforced the hold that the biggest U.S. airlines have on air travel, providing an edge over smaller or low-fare airlines, some of which are losing money. The loyalty programs have helped those large carriers attract and retain customers, who often book with their favored airline because of the perks even if they sometimes pay more.

But the cards that underpin those programs have also attracted critics. Restaurants, retailers and some lawmakers want to overhaul and effectively lower the fees that make loyalty programs particularly profitable for U.S. banks and airlines. For now, those efforts at change have had limited success in Congress.

American Airlines started the loyalty arms race in 1981 when it created its AAdvantage frequent-flier program. United announced a similar offering, MileagePlus, within days, and many major competitors soon followed suit. Over time, the rewards began to behave more like currency as airlines issued branded credit cards and sold loyalty points to other businesses.