Why can the first parameter of fetch() be a Request object?

From the API documentation

This defines the resource that you wish to fetch. This can either be:

• A string or any other object with a stringifier — including a URL object — that provides the URL of the resource you want to fetch.
• A Request object.

Simple question: “why”? The second parameter is init which seems to have all parameters that will be used to then make a Request object.

So:

• Why giving the possibility of using EITHER a Request object as first parameter, OR an URL as first parameter and init value of a Request object as second parameter?

• Is it allowed/spec’d to have BOTH a request object as first parameter, and an init object as second parameter? I assume the answer is “no” since none of the properties of the request object are writable… but in that case, should the documentation above specify that it’s either a request object, or a URL + init parameters?