If you want to have your articles ranked in the top positions, placements of your keywords and their density are essential elements of your article optimization. Due to the way spiders scan a page, it is important to position your keywords where they will be detected and identified as such. As a result, if someone types in that keyword, your article will be shown.
Generally, a keyword should be positioned throughout your article a few times and not just once, as that would basically make every word a keyword. Once a spider recognizes a word put repeatedly in an article, it will conclude that your page may be of some benefit to people who search for such a keyword.
from http://www.seobook.com Tips on how to do keyword research for search engine optimization to help ensure your site ranks well in search engines like 
