Because checking in online beats waiting in line in the counter or dealing with a buggy kiosk. This is a service provided by the airline because if they dont do it, they will lose customers to their competition. That’s the airline’s incentive right there.
The buggy kiosks are becoming a serious problem at many airports, and the airlines seem to be in no hurry to fix them. At Orlando two weeks ago I would be surprised if one out of three worked, and at Denver three weeks ago, ALL, yes ALL of the Southwest kiosks said “please try another kiosk” or words to that effect. WTFH?
On Southwest, I suspect a big incentive is to appeal to business travelers, who would likely be cutting it closer than leisure travelers, and thus get a worse seat by getting a higher number. Checking in on-line lets them get a good number.
I suspect it also relieves congestion around the kiosks, encourages people not to go to the counter, and increases the chances that people will get to the gate on time and not hold up a flight by eliminating one step of the process.