Keyword and Key-phrase optimization is very important, but it's a waste of time if you optimize for words that nobody searches for -- this tool will suggest an array of possible search terms used by surfers, you should then optimize your site accordingly, greatly increasing your traffic. Keywords and phrases are listed in order of popularity (in descending order), so concentrate on the top ones, but look out for some golden nuggets down the bottom which may be much easier to optimize for (due to less competition) yet still yield good traffic.