What do you use the different types of graphite pencils for?

I have always used graphite pencils for whatever felt right at the time, but I'm looking for some things to try out to see if I can get a bit more control over them. What do you use the different lead hardness for? i.e. which would you use for outlines, which for hair, which for tiny details, etc. I know that it's art and not science, so I'm open to everyone's ideas and I'm not looking for a set-in-stone guide.