Value sketches, compositional studies, and colour notes can have a marked effect on a finished painting. It provides time to work out those issues which may be causing problems technically or just offer alternative solutions within a given composition. I am always amazed by John Constable's full scale mock ups of his exhibition pieces. It's a mark of his ability that he was able to carry out these studies with no apparent loss of enthusiasm for the final work. The ideal, I believe, is to use priliminary studies but allow space for improvisation during the final painting. Leaving room for vitality and the unexpected. Hopefully surprising myself and the viewer in the process.