Sometimes when accepting registrations, you want a specific piece of information that will be the same for everyone. Allowing the administrator of the event set up a default value for an attribute, which then does not need to be displayed to the end user, would simply matters. 

Example of use: we require region specifications in attributes for all our RE constituents, and have 9 regions. Asking visitors to pick their region is confusing for them, but necessary for us. When records come through to RE, we manually and individually have to provide them with a region, as it is a required field necessary for our operations.