This question borders on tone reproduction, but to keep things simple, perhaps a helpful way to think about it is that the film is used to record information from the original scene. In that sense, you only need to use enough of the film's potential density range to record the desired detail in the original scene. Whereas when it comes to the paper, one is usually trying to use the maximum density range the emulsion/paper has to offer (white to maximum black). Of course this is a great oversimplification and there are no rules, but for the sake of generalization it may (or may not) help to think about it in this way.