Another reason is notifications. They eventually could notify you on mobile of offers/deals/sales and have a direct channel.

In general, users rarely download apps and most users only have about 20 apps on their phone that they use with any frequency. If an airline can get in to become one of those 20, then they have your mindshare.

I'm a 20-something and I remember thinking I'd never book flights through an app, that's too crazy, and lo and behold I do it every now and then. Once the app's on your phone, they can drive a lot of behavior from usage/features