OK, so you have Tableau licenses for your Creators, Explorers and your viewers.
You love having no limitations on the server capacity to serve your analytically minded users.
However, you are not able to easily charge back each BU. Having site level quotas you help you manage that. For now (as recent as 2019.1), Tableau doesn’t support Site level quotas.
Here is a working solution that you can use to administer the site user counts and quotas for now