The settings page is important to the functionality of the app. Before you can use any sections past the Daily Achievements sections you must generate an API token.
You can create an API token by visiting https://account.arena.net/applications.
Make sure to give it permission to builds, characters, progression, wallet, builds, account, inventories, and unlocks.
You can enter the key by copying and pasting it onto the settings page or by scanning the barcode.
The Clear Cache button will remove all API cache, so in this way you can reset the whole application without individually refreshing each page.