A reason that a site could have been moved over to mobile-first indexing is that its structured data and rich results could differ between its desktop and mobile pages. With this addition, you can clearly see how Google sees this markup on your pages when it’s accessed via desktop Google or mobile Googlebot.
Google wrote, “The new functionality will help you review your structured data implementation using both user agents to prepare for mobile-first indexing.”
Here is the announcement and screen shot from Google:
Since Google is defaulting indexing to mobile-first indexing, beginning July 1st, and with over half of indexing done over mobile-first indexing, Google is giving more tools and advice and what problems a site could have between its desktop and mobile version, as well as help site that have been moved already monitor the impact of changes in the future.