This refers to a use case such as anonymous access. So you could have a Qlik Sense site available for your internal employees, whereas they authenticate and can receive a professional license to unlock all the features. While you can also have anonymous/un-authenticated users who can use analyser license to view and explore.
The bottom line of this is: You need to be authenticated to be able to use a professional license. If you are not, you can only use analyzer/capacity license types..