I think it feeds two main bits of the human brain:

- I'm getting something for free (never mind YQ)
- I'm more important than others (because of the colour of my card)

Those two things make people feel good about themselves. Consequently they are pretty strong drivers of human behaviour and ensure people ignore the rational side of their brain. I'm just wondering if airlines designed the schemes like that or if it was a fairly random discovery they are now glad they have.