The office-js API for add-ins provides a method to get a token that can be used for authorization on EWS or Outlook REST API calls - getCallbackTokenAsync. https://docs.microsoft.com/en-us/javascrip...
Graph has access to other/more/different Event attributes than the JS API does so this would give people a quick way to work around the JS API limitations. This would possibly also address some security issues where App Registration permissions are not granular enough but getting an Impersonation token would inherit user-level permissions.