Managing multiple domains requires being able to add users to each domain independently. Even if you are handling several accounts, Google Domains and G Suite make it easy for you to go to the right domain and add or remove users, as needed.
A large business usually has different departments and teams that work independently of each other, even though they all form part of a larger parent company. As a Google administrator, one of your duties is to help these organizations be able to divide users into the different teams in an efficient way.
