It is hard to say, but a lot of the time, police use the possibility that they might find paper evidence in order to get a warrant to search for papers.  In reality this is done to allow the police to look anywhere a paper might be hidden.  If they had a warrant to look for cases of R&R, they could only look in places where they might find a case.  Looking for a piece of paper lets the police look anywhere (a paper might be).  Of course, anything they find can be used as evidence even if it is totally unrelated to the initial warrant.  Basically searching for "papers" is often done as an end run around the 4th Amendment.