fetch API 中的 request.mode 有什么意义，尤其是在 cors 方面?

Looking at the new fetch API, you can specificy a mode field in the request. From Mozilla:

The mode read-only property of the Request interface contains the mode 
of the request (e.g., cors, no-cors, same-origin, or navigate.) This is 
used to determine if cross-origin requests lead to valid responses, and 
which properties of the response are readable.

然后是如何使用它:

var myHeaders = new Headers();

var myInit = { method: 'GET',
           headers: myHeaders,
           mode: 'cors',
           cache: 'default' };

fetch('flowers.jpg', myInit).then(function(response) {
  return response.blob();
}).then(function(myBlob) {
  var objectURL = URL.createObjectURL(myBlob);
  myImage.src = objectURL;
});

我不明白您为什么需要具体说明在请求本身中如此不言自明的内容?如果我从客户端网站 http://foo.com 服务器请求资源/client.com" rel="noreferrer">http://client.com，服务器没有源头可以看吗?模式不会只会引起混乱吗?