That's NOT a "QR" code on these stamps. It is a 2-dimensional barcode matrix, but not of the QR variety. QR codes have "squares" at 3 of the corners and if photographed by a smartphone will take you to a website. The codes on these stamps are simply the serial number of the stamp plus some other information (in theory the ability for the USPS to figure out who actually purchased this stamp based on the transaction information since that can be tied back to the credit or debit card used at the APC machine. So taking a photo with your smartphone of this barcode matrix will not work since it's not a QR code.
I don't know....I assumed that it's for the Post Office to scan that validates the postage. Not sure though. I tried Googling it and didn't find an answer. :)