Webmaster Tools is a set of tools for webmasters. If you use Google Webmaster Tools together with Google Analytics, the results will be even better. You can access your Google Webmaster Tools data directly from Google Analytics, so if you are already using Google Analytics, with just some more effort you could easily integrate it with Google Webmaster Tools, too.
Google Webmaster Tools is a really valuable instrument for SEO. It gives you at a glance data about all important SEO aspects of a site, such as keywords, links, crawl errors, etc. If you don't use it already, take the time and get familiar with it ‐ it will help you get better rankings for sure.