Schema.org is a joint effort between Google, Microsoft, Yahoo! and Yandex (all search engines) so that is going to be the primary use-case. But their FAQ section says other non-search-engine organisations may join later.
JSON-LD[1], a way to express hyperlinks (linked data) in JSON documents, strongly encourages the use of shared vocabularies instead of every API being their own snowflake with their own private vocabulary every developer need to invest a lot of time into understanding. Schema.org is one such vocabulary.
Not having to read documentation to understand whether the "table" property in the JSON document means "coffee table" or "tabular data" is going to save developers worldwide umpteen man-years. Still, most API developers continue creating snowflakes with proprietary designs, protocols and vocabularies.
JSON-LD is a large step in the right direction by making it easy to share vocabularies. Hydra[2] takes the next logical step by standardizing the protocol as well, so no matter what your API does, it has one standardized way to express it.
Many recipe websites use it. Allrecipes.com has it buried in the tags. One interesting thing about it is the inconsistent application of fields. Units of measures are sometimes cracked out into their own fields. Other sides clump them together. To fix this, we need to use a bit of NLP.
