Example:
Size of POST-request: 10 GB (e.g. HD video)
Upload-time: 4h
The front-server proxies the request to the upload-server.
After the upload has finished, the uploading client wants a quick "all fine"-message (maximum 30 seconds).

If the upload-server gets the stream directly, the machine has has 4h to parse the request (e.g. "upload-module" or "apache-fastcgi-module"). And this does not overload the system.