http - Why use output buffering in PHP?

I have read quite a bit of material on Internet where different authors suggest using output buffering. The funny thing is that most authors argument for its use only because it allows for mixing response headers with actual content. Frankly, I think that responsible web applications should not mix outputting headers and content, and web developers should look for possible logical flaws in their scripts which result in headers being sent after output has been generated. This is my first argument against the ob_* output buffering API. Even for that little convenience you get - mixing headers with output - it is not a good enough reason to use it, unless one needs to hack up scripts fast, which is usually not the goal nor the way in a serious web application.

Also, I think most people dealing with the output buffering API do not think about the fact that even without the explicit output buffering enabled, PHP in combination with the web-server it is plugged into, still does some internal buffering anyway. It is easy to check - do an echo of some short string, sleep for say 10 seconds, and do another echo. Request your script with a browser and watch the blank page pause for 10 seconds, with both lines appearing thereafter. Before some say that it is a rendering artefact, not traffic, tracing the actual traffic between the client and the server shows that the server has generated the Content-Length header with an appropriate value for the entire output - suggesting that the output was not sent progressively with each echo call, but accumulated in some buffer and then sent on script termination. This is one of my gripes with explicit output buffering - why do we need two different output buffer implementations on top of one another? May it be because the internal (inaccessible) PHP/Web-server output buffering is subject to conditions a PHP developer cannot control, and is thus not really usable?

In any case, I for one, start to think one should avoid explicit output buffering (the series of ob_* functions) and rely on the implicit one, assisting it with the good flush function, when necessary. Maybe if there was some guarantee from the web server to actually send output to the client with each echo/print call, then it would be useful to set up explicit buffering - after all one does not want to send response to the client with some 100 byte chunks. But the alternative with two buffers seems like a somewhat useless layer of abstraction.

So, ultimately, do serious web applications need output buffering?