Google Webmaster tools is useful for checking that your site is being indexed properly, looking at back-links and identifying 404’s, but the top ranking search stats have never been particularly useful. A recent upgrade has seen a graph and table of ranking and click through stats added, which is actually very handy.
It shows how successful particular pages are at achieving impressions and perhaps more importantly, the conversion rate. This allows you to optimise 2 areas:
1) If a given page is not appearing as high up or for as many impressions as you would expect for a given keyword/phrase, you can fine tune the page content and get very specific feedback about how that page is performing as a result for the keyword in question.
2) By experimenting with your page title and meta description, you can attempt to make a given page more appealing for users under a keyword or phrase that’s proving more popular than you first thought and perhaps you didn’t optimise for in the first instance.