This is an example of a two tone Underpainting where it was extremely necessary. I felt like there would be no way for me to keep track of my distant trees without a basic guide layer down first. This gave me a clear idea of my values before I started and also acted like a map for me to keep from having my trees get lost in the forest (so to speak).  You can see that an Underpainting can be quite vague or extremely detailed. It is very interesting to play around with.