I carry cards for US and AA (my two most frequent carriers) to use in e-ticket kiosks. Although you can use any credit card, using your FF card sometimes saves the extra step of entering your FF number to confirm your account.

If you put your credit card in a US kiosk, for example, you have to enter one additional piece of information -- either your FF number, your flight number, or something else I forget. If you put in your US card, on the other hand, you go straight to your reservation.

Also, as mentioned earlier, flashing your card is the easiest way to prove you're entitled to be in the preferred line (although truthfully, I have not once been challenged on why I was in a preferred line).