As Mike said, you can switch companies as an admin with a simple dropdown at the same URL. Each company is very much separate in terms of the UI, so you'll only see Company A when Company A is selected for example.
One other major advantage is that your staff members can have different roles at different companies. Let's say you have a staff member named John, and you want him to handle support in Company A, Billing in Company B, full access to Company C, and NO access to Company D, E, and F. You can do that, and they can switch between companies they have access to, having just those ACL permissions you set for them, and the permission options are extensive.
This goes beyond what most people see as "multi-brand" because of the separation and ACL permissions. When you are "in" a particular company, you see only that companies clients, services, transactions, reports,.. everything. And, you can customize your dashboard, and billing overview widgets separately too.