For international flights, they traditionally scan your passport at the check-in counter. As a US citizen, this is mostly just to validate any visa requirement

For international flights, they traditionally scan your passport at the check-in counter. As a US citizen, this is mostly just to validate any visa requirements for the country you're traveling to. However, for non citizens, this data gets sent to CBP to create an exit record. I believe the concern is that between the check-in counter and the flight, you could trade places with someone to effectively overstay a visa without CBP's computer systems noticing (since you'd have a valid exit record)... hence the push for facial recognition at the gate.