When do you need a Knowledge license?

A knowledge license is assigned to a user when the 'Knowledge User' checkbox on the user record is enabled. You only need this to enable the user to create articles, publish them and manage article types etc. In addition to this checkbox, they would need additional permission on the profile as shown here

https://trailhead.salesforce.com/en/content/learn/projects/set-up-salesforce-knowledge/set-access-for-lightning-knowledge.