Increased Airline Website Sales
The trend among some carriers in recent years has been to drive more consumers to their own branded sites, and the weapon of choice in this war for brand loyalty has been promotion or discount codes, often referred to as promo code fares. They require you to insert a short sequence of letters and/or numbers when booking, and in many cases they're the keys that can unlock the absolute lowest fares. For airlines, it's all about driving online traffic back to their own sites. In the past, they often did this by offering bonus frequent flier mileage, but now it's usually done with lower fares
Reducing Airline Costs
You will see major carriers offering promotional fares and in addition low-fare airlines have employed such marketing tools. This isn't surprising, since one of the ways low-cost carriers stay low-cost is to reduce their distribution expenses. And that means not paying commissions to both online and offline travel agencies, not paying fees to third-party booking sites and trying to reduce the expense of maintaining reservations centers. The bottom line is that the absolute cheapest way for a given airline to sell a seat is through its own website, and that's exactly what some carriers are focusing on now.