The Spanglefish 3 system has been built to allow developers to create sites on it for their clients.
Sites can avoid having any mention of Spanglefish in the admin system, payment info can be shown only to the Developer and login screens can be branded with your own logo.
You're a small website developer. You've got a client who needs a simple website. Your client expects you to do all the design work, initial setup and will often ask you to help them with future updates to the text and images etc.
You'd like to use Spanglefish 3 to create the site, but you'll understandably be charging your client for your time and expertise on top of whatever Spanglefish fee is due and your client doesn't need to know what the CMS is that their site uses.
So perhaps you charge your client £500 for setting the site up and £250 to 'host' the site and do any updates they want each year. They already have a domain, so the annual cost to Spanglefish would be £78. You'd be the Invoice contact and would pay the £78 annual fee - your client would never hear from us. They'd never see anything mentioning Spanglefish on the public side of their site. So far so good.
White labelling means that you could give them access to edit the site too. They'll never see any mention of Spanglefish within the admin system, and the login screen will have your logo, not ours.