Using GET with a Body doesn't work if you try using it in the browser with JS fetch for example[1]. Additionally, a lot of existing web servers by default ignore GET requests with a body.

The use case of QUERY is because POST conveys non-safe, non-idempotent requests which can potentially modify stuff according to the REST spec. GET requests on the other hand convey retrieval of a resource. However, due to GET requests not having a body, there's a limit to the amount of data you can put in the URL and you also cannot put sensitive data in it.

Additionally, GET requests are meant to be highly cacheable by default while a lot of the QUERY type requests are usually meant more for one-shot access.

QUERY is meant to address these limitations.

[1]: https://github.com/whatwg/fetch/issues/551