Doing a smaller study helps me with the drawing and details of the larger work, as well as the value scheme. As I do the study, I become more acquainted with the territory of the image-- that is, my mind better knows what the subject really is, and it helps me have a more confident brushstroke as I paint.
From ArtLex, the online Art Dictionary:
value - An element of art that refers to luminance or luminosity -- the lightness or darkness of a color. Value is an especially important element in works of art when color is absent. This is particularly likely with drawings, woodcuts, lithographs, and photographs. It is also true with most sculpture and architecture.