This is very limiting as it prevents you from talking directly to a third-party API without having to use a local proxy service or a technique like JSONP that only works for GETs.
This is usually cleaner than JSONP, and it means that you can do things like set custom request headers and use methods other than GET.
The following form will allow you to use OAuth 2 to grant access to your YouTube account, and then upload a new video to YouTube.
One of the steps involved in uploading the video is sending a HTTP POST to the YouTube API, and before CORS support, this needed to be done server-side.
It's now possible to do all this client-side!