Why is internet required for this add-in?

All new Office add-ins require an internet connection. They are all defined and installed by a manifest file which specifies the URL location of the web source files. These are always hosted on the provider's web server and are not cached for offline use. The licensing system used by most add-ins requires a connection to the provider's web server which in turns needs to verify the user's license against the Office Verification Licensing Service.