value study. Value refers to the levels of light and dark, and have nothing to do with color. Many people do pencil sketches, but I like using a single watercolor pigment. A dark blue works well since the same pigment can be used to represent both the lightest and darkest values needed for the sketch.
The results look similar to an 'open grisaille'. A grisaille is a monochrome painting, and open means the white of the paper shows through representing the lightest areas of the painting. Grisaille are often done as underpaintings for oil paintings.
For me a value study is an opportunity to take a closer look at each part of the sketch and see how it all fits together as a whole. The study should accurately represent the values in relationship to each other. Doing this sketch I noticed how the door and some of the trees were as dark as the underside of the bridge. It can also help you see what needs to be highlighted or darkened to improve the composition. It might even help you decide on color choices for the painting.