What is an opaque response, and what purpose does it serve?

I tried to fetch the URL of an old website, and an error happened:

Fetch API cannot load http://xyz.
No 'Access-Control-Allow-Origin' header is present on the requested resource.
Origin 'http://abc' is therefore not allowed access.
If an opaque response serves your needs, set the request's mode to 'no-cors'
to fetch the resource with CORS disabled.

I understood the message, and tried to do a request that returns an opaque response:

fetch("http://xyz", {'mode': 'no-cors'})

Ok, it now works… but I can’t read it. =\

What’s the purpose then, of an opaque response?