For some buffers, a list of files and tags makes no sense. This could be because files are not currently in reference (such as web pages), or that the files you might be interested have special properties (such as email folders.)
In these cases, a minor display mode is needed. A minor display mode will override any major display mode currently being displayed for the duration of the specialized buffer's use. Minor display modes will follow the general rules of their major counterparts in terms of key bindings and visuals, but will have specialized behaviors.