If two different RDF files (eg. FOAF documents) are talking about the same thing but don't use exactly the same URI when mentioning that thing, how are our poor stupid computers supposed to be able to understand? In the real world, we want to write RDF documents (eg. for FOAF) about things that we've not yet agreed on common identifiers for. This is one of the core problems we've had to address in FOAF.
http://example.com/food#egg. I know that. You know that. Computers are too stupid to figure it out without a high degree of hand-holding.