If you've made updates to pages in your website and you would like those changes to be updated with Google, the Fetch as Googlebot Tool can be a very useful tool. It's also a good tool if you have added new URLs (pages) to your website. Another example is if you previously published content you didn't mean to or incorrect content and would like Google's index to be updated with the new, correct content.
One of the great features of this tool is that it lets you see your site as Google's search bot (Googlebot) sees it. For more specific information about the information provided by Fetch as Googlebot, please see Google's help documentation and official blog:
Begin by logging into Google Webmaster Tools and selecting your site.
In the Google Webmaster Tools dashboard, Open the Diagnostics Menu on the left side of the screen.
Click on the "Fetch as Googlebot" link.
Finish the URL in the text box provided for the specific page of your site. For example, if I wanted to use Fetch as Googlebot on the default.htm page of whhsupport.com I would finish the url with default.htm.
You can also choose to fetch your URL with Google's different bots. Select Web for the regular web crawler Googlebot view. If you would like to see how Googlebot-Mobile sees your site, select either XHTML/WML (for most sites), or cHTML (usually for Japanese sites).
Click the "Fetch" button.
If the request is successful, you will see "Success" under the "Fetch Status" column.
If there any issues, you may see a message such as "Not Found" or something similar. Regardless of the status, you can click on this status link to view how Googlebot sees the page you submitted.
As long as the fetch was successful, you can submit the URL to Google for re-indexing. When you submit the URL to the Google index, you will have the option to submit just the URL or the URL and all pages linked from it.
Please note, Google does have a limit of 50 submissions per week for just the URL. If you are submitting the URL along with all pages linked from that URL, the limit on submissions is 10 per month. You can view how many submissions you still have available on the Fetch as Googlebot page.
While there is no time frame given by Google to index the URLs submitted through the Fetch as Googlebot Tool, it may help to speed up the indexing of new or changed pages on your site.