I have learnt the importance of outline hard way. When I first started a novel, many years ago, I started writing whatever scenes came to my mind. That is the approach I used up until recently. It worked in terms of the scene that just happened to land smoothly in my brain, but eventually I would find all these unfinished pieces of work with gaping holes in the middle.
What happened when I tried to fill those holes? I either had to change other information in completed scenes, or the holes remained because I didn't know how to bridge the gap from one scene to another.
Finally, I learnt my lesson. Inspirational writing is all very well, and I still adhere to it. But now, I keep a seperate file called, "Misc. Scenes" where all these random scenes go. If they happen to fit in the outline, great.
I am still polishing this approach, trying to streamline my outlines, because I often find that once I get an idea, they start running wildly. And before I know it, I have more information than I can put in a book.
So now the next step - a very hard step - is to slowly write outlines for half-completed novels, ideas, and then I will most likely have to rewrite everything. But better to do it right then to produce less than what I am capable of.