The importance of doodles: I am a great believer in doodles. I am a notorious meeting-doodler, and believe there should be more doodles in the world. I carry around a Moleskine for this purpose, and here are some examples.
I do not start with an image in my head of what I want to accomplish. There are two ways of getting started; one: begin with a small, well-defined shape and then build from that. The idea is contrast in shape and density. However, a little planning is a good idea. Some parts will be in front of others. Two; start with a line. Often a curvy one, but ones that stretch over the paper. Then add contrast and texture.
As for figurative doodles, realism is overrated. The point is to play, and to get back into pen, pencil and paper.
You can find my answer to this question on StackExchange Graphic design.
You can see more of my doodles here, and you can, in fact, buy some stuff with some of my doodles on it from Cafepress.