Based on my personal experience as EXP, I find both the membership ID and Luggage Tags are useful. I prefer to print out hard copies of my boarding passes (as a backup to mobile passes). I swipe my EXP card in the credit card slot in a self-service terminal to get started. It saves time. When I fly to Europe in Premium Economy or Economy, the card is helpful to enter the first class or business class lounges (I flash the card before scanning/handing-out my economy bp), and more importantly, to access the first/business check-in/cs/immigration/etc. lines (gatekeepers waves me in as soon as they see my card). etc.