I'm elite on SouthWest and Delta. I know SouthWest gives me a free ticket for my wife every time I book a flight. So I effectively have double the ticket value now. They also have shorter lines and earlier boarding (which on SouthWest means you can choose your seat, the seat or two with a blank space in front of it being the best choice) so I don't have to go to the airport as early.
Delta, similarly, gives shorter lines, free upgrades to first class based on availability (you can always get them if you do something odd like take the first flight out of Newark before the buses are running), and free standby and same day confirmed flight changes (just book the last flight of the day, go to the airport whenever you want, and use your free change). Free standby is really handy for illegal transfers too. E.g. Fly to LA then hop a plane leaving immediately - officially you can't do it because there has to be a certain time window for you to transfer, but with free standby they'll put you on the plane as long as you are at the gate and there's a seat.
So a lot of times you get things that would cost a lot more (companion ticket, first class, etc.) and other times you just get some of your time back (shorter lines, illegal transfers).