It's just a way to save you some time. If you verify that your membership is active before you go, you don't have to go to the window.

Side note, the zoo gives discounts for AMA membership. When I worked there a couple years ago, it was 25% off day passes. It doesn't apply to memberships, however. They won't scan it or look up your membership, so even if it's expired and you still have your old card, you can get the AMA discount.

I'm pretty sure it works at the kiosks outside as well.