While the tool is a great reassurance that Google is indeed crawling your site, it also can be used to identify and troubleshoot problems. From Google’s Webmaster Central Blog (http://googlewebmastercentral.blogspot.com/2012/07/behold-google-index-secrets-revealed.html):
“However, some of you may find issues that require looking a little bit deeper. That’s why we added an Advanced tab to the feature. You can access it by clicking on the button at the top, and it will look like this:
The advanced section will show not only totals of indexed pages, but also the cumulative number of pages crawled, the number of pages that we know about which are not crawled because they are blocked by robots.txt, and also the number of pages that were not selected for inclusion in our results.”
The tool provides a rare look behind Google’s indexing curtain, and a good way to make sure that all of your content and SEO efforts are not in vain.