If the airline pays an employee a dollar, it costs the airline a dollar (actually more with employment taxes, etc.) If the airline gives the employee a dollar's worth of free travel (based on ordinary ticket prices), it costs the airline a lot less than a dollar. At some level, at least, the labor market is competitive, so an airline that gives staff free travel is likely to have lower ticket prices than one that doesn't. Within reason of course -- a job with no pay but unlimited travel privileges wouldn't be very attractive (though if your spouse had a good job . . .).