Any RDF document could give an ID to its described literal or resource. That means that you could have multiple resources with the same content. Lets say we have an application that lets you update a short description about your intrests. This discription could be a literal or a resource. The answer to this is up to the application.
Some application wants to handle version history.
Other applications want to make sure the value doesn't change. But many applications probably wouldn't care and would like to just change the text value, without having to create a whole new object. Literals are data. Most of the time it's textual data. If it is a short piece of textual data, you would like to inline it in a xml representation.