They want to make sure you're on the right flight (some news stories about people on other Korean airlines getting on the wrong flight and wrong airline all together, like they swapped boarding passes two people). Also they want to guarantee that you show up - they have a new no show policy and penalty because of Kpop fans getting on flights and de-boarding after taking pics (costly to fix that mistake) or just getting a ticket to follow the stars around the airport and not take a flight.