The AC card is like an addon card for yourself.
Let's say if you want to separate certain expenses or give it to a younger brother and pay all the expenses he makes on your card.
The AC card comes in very handy for things like that.
You can keep a track of how much expenses are being made on that card, set limits and pay the bills directly.
(The bills are shown separately but are added to your Gold Card billing cycle so that you can pay the bills for both cards together in one go)