There is more to it than that. It provides an expressive syntax to construct all sorts of requests. You can submit forms, upload files and set headers without having to use flags. You also get syntax highlighting, ability to pipe in request data, etc.
If you are sending complex JSON, it's probably stored in a file or it's the output of another program:
http PUT httpbin.org/put @/data/test.json
http -b localhost:8888/couchdb/ | http PUT httpbin.org/put