significance of client_body_buffer_size as opposed to piping to upstream right away?

I am trying to understand the reason for buffering incoming request (client_body_buffer_size) in which nginx would either keep request in memory or write to file client_body_temp_path (based on request size).

What are the performance advantages and/or technical challenges to such an approach as opposed to piping directly (even for smaller requests) to unbuffered piping to remote server e.g. http://tengine.taobao.org/

Please share your thoughts.

Thanks