For larger companies with a larger number of Tableau Online seats, there are often teams using the same site. For instance, one team from HR, one team from Marketing and one team from Finance, however, Finance doesn't necessarily want to share all their mid-quarter data with HR, Marketing and whichever teams may happen to be on the site. Now, they could potentially just purchase a separate site, but when they eventually want to share with the rest of the company the project they have been working on, there is a lot more work involved with uploading to the company wide site.
If we could have a password protected setting on workbooks for teams that want only their team to be able to access (yes, including site administrators) because of team sensitive data and projects, I think that would add a next level to Tableau Online. Why should every site administrator, like Jack from HR, Suzy from Customer Service and Beth from Finance, have access to each other's workbooks on a paid site?