Contrast and ISO analysis is all 0.1 and above, but.... for the purposes of push processing or extracting details down there, it's useful to see where there's density below 0.1 and where the film just plain stops responding at all. There is actually a point where the film just won't respond even though you've exposed it to light, and that really low density, low contrast stuff down there can be brought up a quite a bit through development without completely blasting out the highlights and depending on what you're planning to do with the negative, if digitizing it, that bottom 0.1 density will be digitized with at least a couple thousand discrete tone values. You can really stretch that 0.1 out quite a lot after the fact if you want to, which is why it's helpful to know how many stops are registering down there before the film stops responding to any light at all. Obviously, if printing in the darkroom, pretty much anything below 0.1 is basically useless and you want to expose so that zones 1-10 are on the straightest part of the curve, but if going the hybrid route or if you shoot in low light a lot, it's helpful to know where the film goes kaput and doesn't register anything.