On one of my other blogs SEO Made Easy, I have outlined a rather involved and perhaps tedious process to brainstorm, organize, expand, and refine a websites keyword list. Lord have mercy the process looks daunting -- even to me.
Question: So why go through all of this effort to find keywords and keyword phrases?
Answer: Because you are looking for lightning to strike.
You are hoping that somewhere during the process of hunting up keywords you will stumble across those total gems that will completely alter your website and marketing of it.
I'll give you an example. A friend was working on his website devoted to astral projection. When going through the process of creating a list of keywords he was suddenly reminded of the phrase astral travel. This might seem like such an obvious phrase to include. But, for some reason he was temporarily blinded to it. Doesn't matter why. What matters is that through the simple process of gathering keywords he was in fact reminded of it.
This may seem like a small thing. However, when you consider that 30% of the traffic in this niche is generated by interest in astral travel, you realize how important this little reminder was.
Perhaps not the most dramatic example. But, it should give you the idea of what is meant by looking for lightning to strike.