The idea is that you want to combine resources from multiple documents, in order to achieve something.
You could either start the discussion based on multiple documents, or start based on one and later add more in a session.
Maybe for cross-referencing, or asking questions that involve combining information residing in different pages.
Maybe also another far-fetched scenario that probably stretches the definition of a search engine: providing some technical specification, and the documentation of a programming language or framework, ask it to implement something.
