The OP already clarified this somewhat...it's common for those photo ID cards you are thinking of to be called "badges". The OP is not running around with a shiny metal badge that says "US Customs" as I think you imagined, so there's no need to have law enforcement grill him!

Here's my guess as to the 'meaning' of the badge--remember that before you clear customs, you are not considered to be in the United States. The customs area is thus a more secure area than an airport in general (at least as far as who is supposed to come and go). The "badge" is probably needed because only certain airport employees who are deemed to need access to the customs area are allowed in there. If this is correct, it should not allow expedited trips through customs when traveling.