Basically it allows you to have fully "as intendeed" Google Calendar experience without launching the browser. Every other calendar app has shortcomings for an instance Apple Calendar is fetching data with CalDAV vs direct API connection that allows you to i.e. create Google Meet inside the app, color events as you want and every change is visible on Google Calendar, etc.
Native ASWebAuthenticationSession for the OAuth flow with PKCE (SHA-256 challenge). Refresh tokens are stored in macOS Keychain (kSecAttrAccessibleWhenUnlocked), access tokens are never persisted — refreshed on demand. No custom WebView, no embedded browser, no token stored in UserDefaults or on disk. The only things in UserDefaults are the account email list and display names.