Just to join? Costs nothing. And, technically, it allows you to use discount codes for car rentals and other things (though I've never been asked to show my ff number when renting). If you're talking about a credit card to earn miles, which costs money, then that's another thing. That is how you earn miles from purchases. There are some cards that allow you to accure miles and then decide later how to use them (e.g., which airline).