There's a lot of competition to get good spots in the search engines for some specific keywords or key-phrases, so proper html syntax and a clean code help toward better rankings. Let's say we have two competing sites, both are suitable for the top spot and the search engine has a tough time deciding who to put first. Chances are that the validated site with proper syntax will be the winner. Validation of the html code on your site alone will certainly not guarantee or secure you a high rating, but as with so many other things it will help and might even be the contributing factor to get you across the line.