Sending a URL encoded form or some JSON in a POST request is also easier for most people to understand than the myriad ways you might format a query string in the URL (which may have a stricter limit on size).

You only have to look at how different services handle arrays in query strings to understand that serialising it is conceptually easier.

Comes up a lot in search or filter APIs. I'm sure there was some effort many moons ago to create a QUERY method for that.