Search engines have long been scarily good at understanding content on the web, but there's still a lot to be gained by making explicitly clear what you're trying to say and whom it's intended to help. Tools for doing that have existed for decades – meta tags, sitemaps, and even your robots.txt file.
One of the more complex and less understood of these tools is structured data – the specially formatted information embedded onto a page, specifically intended for machines to read.
Until now, those machines have almost always been search engines. And they've largely just used it for generating rich snippets like these: