Google webmaster tool Fetch adds new function Fetch and Render. With this tool you can see how Google bot renders the page. At normal fetch you can see only an output of code and HTML, but with render Google will show a visual representation of what Google bot sees.
Google officially announced through their Webmaster blog says “In order to render the page, Googlebot will try to find all the external files involved, and fetch them as well.”
Fetch as Google tool provides a way to submit new and updated URLs to Google for indexing. When you submit a URL in this way Googlebot will crawl the URL, usually within a day.
Shimi Salant, a member from Google web Master teams says that ‘Googlebot follows the robots.txt directives for all files that it fetches. If you are disallowing crawling of some of these files (or if they are embedded from a third-party server that’s disallowing Googlebot’s crawling of them), we won’t be able to show them to you in the rendered view. Similarly, if the server fails to respond or returns errors, then we won’t be able to use those either (you can find similar issues in the Crawl Errors section of Webmaster Tools). If we run across either of these issues, we’ll show them below the preview image.”
Google suggests this tool to diagnose a page’s poor performance in search results.