Useful keywords are often chosen through using a Keyword Effectiveness Index (KEI). A KEI compares how many times a keyword has been searched to the search results it generates in hopes of finding out what keywords will be useful for your campaign.
Suppose the number of searches for the keyword “SEO” last month was 1,000 and Google displays 1,000,000 results for it, the ratio for the KEI would be 1,000 over 1,000,000 or KEI= .001. This is the ratio between the popularity vs. the competitiveness for that keyword. The higher the KEI, the more popular your keywords are, the less competition they have and the better your chance for getting to the top using that keyword.
So if people were searching for the keyword “love for search engine optimization” 300 times last month and Google only showed 100 results, your KEI would be 3 and you would have a much better chance of getting ranked higher by putting “love for search engine optimization” somewhere on your site (make sure you are doing it in a white hat way though).
KEI might be useful for gauging what keywords are popular without much competition but it never gauges the quality of that competition. The competition might have a large SEO team and thousands of back links making it look like you might have a good chance of ranking for a word but then never do.
The other problem with using the KEI is how it might not be useful for comparing across all keywords. Since all keywords could be calculated differently and filters can cause huge misrepresentations for some, the validity of the comparison might be called into question.
Overall, the KEI is just one tool in many for choosing the right keywords. The right keywords are a must for any optimization campaign. The more tools you have to help you get the best ones to bring your ranking up the better.