The card shows an address to paste into the app's portal. What is it for?

It tells the app where to send your orders. After you press Connect on Settings → Delivery apps, the card shows that address, and you copy it into the field in the app's own portal that asks where orders should go. Without that step the app has nowhere to send them and nothing arrives. Once it is pasted, send yourself a test order from the app and watch it appear under Orders from the apps within seconds. The Orders from delivery apps tab in the docs walks through the connect steps.