he advantage is that you’re likely to get a better seat if you’ve reserved one you don’t want, or if you don’t have one reserved. It also bypasses the burden of needing to manually input information—like passport info and birthdays—multiple times.

The disadvantage is that you’re giving your personal information to another website and letting an automated service choose your seat. And once a seat is chosen, it usually can’t be changed until you get to the airport.