why multipart/x-mixed-replace is needed for Comet?

I'm reading this article about Comet http://en.wikipedia.org/wiki/Comet_(programming)). It mentions that browser should support multipart/x-mixed-replace to make XmlHttpRequest Streaming possible.

Why this multipart/x-mixed-replace is necessary?

Without this header, HTTP response can still be chunked and sent piece by piece to browser, right?

Make sense to you?

http://en.wikipedia.org/wiki/MIME#Mixed-Replace_.28experimental.29

The content type multipart/x-mixed-replace was developed as part of a technology to emulate server push and streaming over HTTP. All parts of a mixed-replace message have the same semantic meaning. However, each part invalidates - "replaces" - the previous parts as soon as it is received completely. Clients should process the individual parts as soon as they arrive and should not wait for the whole message to finish.