Google: The Submit To Index Feature Is For Critical Cases

In Google Webmaster Tools, there is an option to fetch and render as GoogleBot would crawl your pages. After the page has been fetched, Google will sometimes give you the option of submitting the page to the index.

John Mueller said in a Google Webmaster Help thread that while the fetch and render quota is 500 per week, the submit to index quota is 500 per month.

He also said that submit to index feature is meant to be used on a very limited basis, only in "critical" cases where you need the changes to be "reflected in search faster than usual."

It seems, at least in the thread, some want to use it all the time for new pages added daily and they do not have enough quota.

Forum discussion at Google Webmaster Help.