Is Schema.org structured data (in Microdata, JSON-LD, or RDFa) relevant for SEO?
I know that structured data can provide benefits in other areas, but here I’m only asking about its significance for general web search engines.
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It can enable features in the search engine interface.
Providing certain structured data can give you the chance to get a certain user-visible feature. This can be the case for your page, for your whole site, or for general information not coupled to a search result (e.g., in sidebars, in carousels etc.).
The most prominent examples are rich results. Instead of getting a regular search result (title + URL + text snippet) for your page, the search result can get enriched and contain certain information from your structured data (e.g., a name, an image, a rating, etc.).
These don’t change your page’s ranking, but they can increase the click-through-rate. (They could of course also decrease it. For example, when the rating is shown in the rich result and the user decides not to visit the page based on that rating).
Search engines typically document which features they support and what structured data is required to get them. Starting points from the search engines that sponsor Schema.org:
It could help search engines to understand what a page is about.
The best example that search engines do have a need for such explicit signals (structured data), instead of relying only on implicit signals (semi-structured and unstructured data) are the features mentioned above. If search engines could extract everything they want to know about a page without structured data, they would certainly do it.
So the data is there, and they already make use of it for some purpose, so it’s conceivable that it gets used for learning more about the page content, too.
It would be one of countless signals taken into account, but if used, it could lead like any other to a change in ranking. Up as well as down. Because a better understanding of the page content makes the page more relevant to some and less relevant to other queries.
Matt Cutts (Google) gave an example in 2012: it could help that your page shows up for certain queries in content-type-specific searches. And apart from that, he didn’t take a ranking influence "off the table".
It could help search engines to decide which feature to offer next.
If many sites provide certain structured data, and the quality is good, it becomes more interesting for search engines to make use of it.
So if a big part of an industry starts using a schema, the chance might be higher that search engines start to do something with it (compared to the chance of search engines proposing the use of a schema that isn’t yet widely used). It might still be a low chance, of course.
It doesn’t necessarily increase your page’s ranking (i.e., no: add this, get better rank).
As noted above, structured data could lead to a better understanding of your page content, and that could lead to a change in ranking (not only an increase, but also a decrease) for specific queries. It’s unknown if or to which extent search engines do this today.
(bold emphasis mine)
On-page markup helps search engines understand the information on web pages and provide richer search results.
Schema.org focuses on defining the item types and properties that are most valuable to search engines. This means search engines will get the structured information they need most to improve search.
Dan Brickley (Google, working on Schema.org) gives an "informally Google-oriented perspective" about what search engines might do with Schema.org: public-schemaorg mail from 2017-03-20
[…] gives Bing valuable information on the type of content you’re hosting on your site. On our side, we put your annotations to good use, for example by using them to increase the visual appeal of your search results, or to supplement and validate our data sources.
Google Search works hard to understand the content of a page. However, you can provide explicit clues about the meaning of a page to Google by including structured data on the page.
Google uses structured data that it finds on the web to understand the content of the page, as well as to gather information about the web and the world in general.
Other structured data: You can add additional schema.org structured data elements to your pages to help Google understand the purpose and content of the page. Structured data can help Google properly classify your page in search results, and also make your page eligible for future search result features.