One of the most important objectives for an AdWorks advertiser is to improve its CTR. This is because of the fact that you can pay less per click than ads below yours if you have a better CTR. One way of achieving this is by using negative keywords, especially if any of the keywords phrases in your ad campaign are broad. If the search phrase includes one of your negative keywords your ad will not be displayed. This will help to minimize click throughs that do not convert to sales. Your CTR will thus increase because there will be fewer instances of searchers viewing the ad but not clicking.
For example if you charge a premium for your products and services you can use negative keywords like: cheep, free, discount etc.