Webmaster Tool is a free web service provided by giants like Google and Bing to check the indexing status of a website and optimize according to it.
It helps you submit Sitemap to Search Engines, Check Crawl rate and website Index Status. Also, Helps you create Robots.txt to control Web Spiders from Indexing unwanted pages.
You can find, Broken Links, Backlinks and Organic Traffic along with keywords that triggered your website in search engines.
You can also control Geographical Location that you wanted to concentrate on with Webmaster Tools.
Ther are few key features that help gain more Organic Search result, Structured Markup tool, Email Markuptool and Mobile Friendly Tester. Some of them are new, yet they are very important for New SEO srategy.
I hop you find this helpful