It might not seem obvious, but clicking the "Search tools" button again replaces Google's advanced search options with a line that includes the number of results and the time used by Google to generate the search results page. Clicking "Search tools" doesn't disable the advanced filters, you need to click "clear" to do that.
The number of search results is a very useful information because it shows if there's enough content for your query. If Google only returns 50-100 results, you may need to remove some of the keywords, fix some mistakes, disable some of the filters. It's sad to see that Google no longer displays the number of results in the mobile interface (for both smartphones and tablets) and you need to switch to the "classic" interface to find it.
Obviously, Google can't find the exact number of search results, so it only shows an approximation truncated to the first 3 significant digits, but even this imprecise number is useful.
