Sitelinks also factor into your ad rank, and can influence the position of your ad on search page. Ad Rank previously took into account your max cost-per-click (CPC) and quality score, and now ad extensions and formats are another component of the calculation.

Seems like a no-brainer to implement, right?

Surprisingly, many clients are still not taking advantage of this feature. Google actually noted this in a blog post in January. After some extensive research, I found a surprisingly small number of customers are taking full advantage of the benefits of sitelinks. In the example below, Merry Maids is using the traditional sitelink format; the two other businesses aren’t using sitelinks at all.

http://searchenginewatch.com/article/2332798/Take-Your-Sitelinks-to-the-Next-Level-With-Descriptions