I am confused about the purpose and functionality of Passbook. I have installed it but it doesn't seem to do anything. I put the starbucks app on my phone so I can pay for my coffee using my phone and I stumbled onto this in researching passbook as there seems to be some shared compatability between it and passbook. For the life of me I don't see it. How do I use passbook as an app to purchase tickets, etc? I just don't get it! HELP!!