If you use your account entirely by yourself, you might not need this feature.
Using this function, you can create subsidiary users within your parent account, provide them with certain rights you decide by your own, view user's statistics and fully control them.
For example, you can create subsidiary user which can only create landing pages, or cannot create new campaign but can view the existing ones. You can also provide users with the access to the list of your campaigns or hide your campaigns from them.