Content-type for token request in OAuth2

Question

Getting curious about https://www.rfc-editor.org/rfc/rfc6749#section-4.1.1

Take a look:

The client constructs the request URI by adding the following
parameters to the query component of the authorization endpoint URI
using the "application/x-www-form-urlencoded" format

...and the server has to respond with "application/json".

The questions are:

1. Why should we use explicitly "application/x-www-form-urlencoded"?
2. Are there any obvious benefits from urlencoded?
3. What stops me from accepting both urlencoded and json here? Seems, that I am not really violating RFC this way.

Answer 1

I faced with this question too. I don't know if there are any benefits from urlencoded but big player in this industry auth0.com is using application/json content type for both request and response.