Whether it is to debug the site, or just to see the overall performance.
On the internet where more people are browsing using their mobile devices, having a mobile-friendly website is becoming more than just a must. Google's mobile-friendly tool is to test how easy a visitor can use your page on a mobile device.
"Today, everyone has smartphones with them, constantly communicating and looking for information. In many countries, the number of smartphones has surpassed the number of personal computers; having a mobile-friendly website has become a critical part of having an online presence," said Google
Rich results on the other hand, are search results that go beyond the standard blue link, they may include a carousel, image or other non-textual UI elements. The testing tool focuses on structured data types that are eligible to be shown as rich results, and allows users to test all data sources on their site's pages, such as JSON-LD (which Google recommends), Microdata, or RDFa.
"Over the years, the different ways you can choose to highlight your website's content in search has grown dramatically. In the past, we've called these rich snippets, rich cards, or enriched results. Going forward - to simplify the terminology - our documentation will use the name "rich results" for all of them," explained Google.
To use the tool, users just need to enter a page's URL to see how the page scores.