This feature generates an anonymous, virtual email address when users sign into your app, to prevent you from keeping personal information in your sheet if you want to avoid it. As far as the user is concerned, the app will behave totally normally, but the app developer will only see a virtual email address like chosen-twist-631@anonymous.appuser.io.
We plan to make this optional for paid apps, and mandatory on the Personal plan, to protect user privacy.
If you want users to give you their email address, you could make the app ask for it.
Please try it on staging and let us know what you think!