DFAC scans your CAC's front bar code; the gate scans the back. Any difference?

I'm just wondering why your CAC card has two different bar codes that apparently serve different purposes. At the DFAC, they specifically ask to see the front. But at the gate leading on post, the guard scans the back of the card. Why does one soldier need two bar codes? For that matter, why does every DA form ask for my social security number along with full name when the military assigned me a DoD ID number that should have eliminated the need for me to provide my SSN ever again?