Keyring is another component, specifically for passwords.
GOA is for storing account information: e.g. you configure your Gmail/nextcloud/whatever account once, and then all apps that need it can access it. So your Geary, Contacts and Calendar (separate applications) do not need each to implement account management and account specific auth flows -- not just passwords, OAuth is all the rage nowadays. And I, as the user, do not have to keep tokens in each app current.
It is also not 'entire gnome shell`. Just the required components.
GNOME has a keyring system service for storing passwords, so it's not necessary to get the entire desktop shell to access the keyring.