This add-on requests read access to your Google Analytics data so it can copy it to your spreadsheet (so you can view it).
All add-ons use Google's standard OAuth 2.0 system for requesting access to data via APIs, and you (the user of the add-on) are shown a permission request dialog prompt explaining what access level is being requested and what data that would allow the requestor to see.
If you have additional questions about how OAuth works at Google, you should ask in a different forum, as this group is specifically for questions about this add-on.