Necessities are good for editing purposes as well. I try to ask myself if this scene is necessary (and to remind myself why it is) or could it be bettered? Is the book/story itself a necessity? Before getting too depressed on that point, ask yourself who would read this work? Can you figure out who would like it? If so then there is a necessity somewhere (oh and if you're writing for pleasure only, that is a necessity for you, whether you write for therapeutic purposes, to just enjoy the challenge or whatever).
Necessities can drive a plot too. The Queen, to begin with, is driven by the necessity to not upset Eileen too much, so allows her defection, planning all the time to get Eileen back. Later, the Queen's necessity to get Eileen back becomes an obsession that takes the monarch over and makes her act in a way she would not have done only weeks previously.