In some portal scenarios you might want to have only webmaster(s) manage the layout of the site. So your goal is to avoid that a registered user change it.
With Webparts you might build portal that match this scenario like this:
Add to you web.config:
<allow roles="webmasters" verbs="enterSharedScope"/>
This configuration set that only users with the role webmaster might change the layout.
Then you might place a WebPartPageMenu control on your site. This control is listing all the personalisation to the webpart system you might do as a logged in user. In the case of a user in the role of webmasters, the WebPartPageMenu will have one of it entries set to: Show Share view.
You can select this entry and then Design Page Layout to be able to design the layout of the page for the user the anonymous users.