There's something about air travel that makes it a uniquely terrible experience and drives me to participate in these programs.
There is nothing I do in my life regularly where I'm more restricted, or more closely related to a rigid caste system than air travel.
Every time you take a flight, for somewhere between 3 and 20 hours, you're told exactly where to stand, where to sit, what you can and can't wear, what you can and can't carry.
Almost everything about frequent flyer status is about making these limitations slightly less terrible. You're still going to be stuck in a shitty seat for several hours, but your seat might have slightly more legroom. You're still told when you can get on the plane, but you'll be at the front of the line. You still have to go through an invasive security process, but you'll go quicker, and it'll be slightly less invasive.
Most of these are stupid little things (and there are other much more objectively valuable perks, like waived change fees, improved options in the event of cancellations or delays, free checked baggage and the like), but in the artificial, restrictive and status-driven world of air travel, they're so nice to have.
All that said, I still think Southwest is the best domestic airline to travel and not be treated like a complete peon, assuming you don't fly enough to earn status with a traditional carrier.