Not having a plan is the easiest way to ruin your woodworking project. Some people are really good at shooting from the hip and making it to the end with a good looking project. However, the majority of woodworkers are not like that. Most people are a blend of the two, and can benefit from at least some form of a loose plan before beginning.
My father and I are very different when it comes to making things. If given three days to make a project, I would dive right in on day one. I would end up making the project a couple times, making several mistakes, and wasting some wood. At the end of the three days though, I would have a working project.
On the other hand, my father would spend the first two days planning. He would work out all the details on paper, and discover all the problems before making the first cut. Then, he would go into the shop on day three and make a perfect project on the first try. We both get the same result, but we have radically different ways of accomplishing it.
This is two ends of the spectrum on planning. I tend to plan very little, and I pay for it. I have gotten better over the years, and I now plan more than I ever did before. My father is on the opposite side, and over plans. That is what he is comfortable with, and that is how he ensures that his projects come out nicely. I could stand to get a little closer to his style, as it would save me money on wasted wood.