Django: When to use multiple apps
For example, if you have an admin and a user interface you can separate them as ;

admin app  
user app
When are multiple apps actually used? I've been trying to find a concrete example of when multiple apps might be used, but haven't found anything.
I've been reading through the docs, and following the tutorial, and it says that an app has a single functionality - what does this mean? This is open to interpretation depending on the level of detail: it could refer to the individual components of a blog perhaps (ie. the menu bar, the individual blog entries, the comments section); it could refer to the pages the visitors see, and the pages writers use to create posts; it could even refer to two separate websites running within the same server.
Can someone give an example of a project which uses more the one application?