The Rich Results Test tool (RRT) is the original replacement for the SDTT, which they continue to urge users to switch to using. According to Google, the RRT fully supports all Google Search rich result features.
There are 2 crucial improvements over the SDTT that the Rich Results Test offers:
1. It renders the page content, so it can validate dynamically generated structured data.
2. It allows you to test either as a desktop or mobile user-agent (mobile by default).
This technological improvement also aligns with Google's shift to rendering every page they crawl and indexing content on a 'mobile-first' basis.
The new tool also allowed Google to re-brand, clumping together rich snippets, rich cards and other search results features under the umbrella term 'rich results.'
However, many SEOs were very unhappy about the change, in particular to decision to deprecate the Structured Data Testing Tool.