Use of keywords help the Google Mediabot achieve higher Ad-relevancy for your website.Some people follow a strategy of hosting less content on a web page. This helps focus ads on specific keywords and avoids dilution of ads by other keywords that you may have mentioned even once in the web content. This works when you are optimizing for Adsense, but has some traffic drawbacks too. Furthermore, Google may consider it as SPAM. Therefore, it's never advised to host 2 or 3 lines and expect good earnings.
Sometimes, Ad bots can be very sticky to a certain keyword you used just for a reference.