In spite of recent suggestions by Google that they are clamping down on sites using link spam to manipulate search results it seems the search engine is still struggling to introduce a solution to effectively tackle the problem.
Traffic from Google is an extremely valuable commodity and some sites will stop at nothing to secure top rankings for lucrative keywords. One of the main factors that determine how well your site ranks is the quality of links that point to it. These act as votes for your sites authority and help to improve your position within the search results.