Speaking from the personal experience I found that the last stage which is all about adding the details, working on the contrast and shadows does not help me to improve my skills although it takes a lot of time. The stage before that, when I work on composition, right shapes is the stage I learn the most from. Would you agree?
Maybe landscapes are the best example to see exactly what I mean. Let say we paint a portrait or a city. In both cases if the fundamental part goes wrong the whole final stage is not going to fix it by adding more details over and over again. It happened to me few times in the past that I sketches the basis and I was not acurate. I was under impression its not a problem I can adjust it later. I could cover the mistake in some respect however I was not happy with the final look. Only by changing the mistake completely the whole painting started to look exactly how I wanted. As an artist I want to push myself and improve my skills all the time hence knowing which stage is crucial to make the artwork is essential.
I hope you found it interesting.