Airlines these days are run by a complex set of systems most of which have to work in order to have the airline function. I remember a few years ago SABRE (one

Airlines these days are run by a complex set of systems most of which have to work in order to have the airline function. I remember a few years ago SABRE (one of the 3 major GDS companies in the world) had a 4 hour outage (I worked for a division). Half the world's airlines stopped functioning. Generally these things seldom go down but when they do, hell breaks loose. Often these systems not only do reservations, but crew scheduling, weight balancing, check-in, manifest creation and a whole host of other small but important things. Airlines also integrate some of their own pieces into the contracted ones, and some just contract almost everything. It usually comes down to money. Even Southwest Airlines which doesn't share its res data with anyone uses a GDS backend for a lot of things (in this case SABRE).