It sends content to the client, because when _jspService(...) method finishes, the buffer which was used for that page is flushed to the client.
The purpose of buffer is to accumulate many "println(...)" outputs and then send to the client all content in one action.
The practical use of buffer is: if you have big buffer, and in the middle of the page you realise that you need to display ANOTHER page - you can forward to another page and since nothing was sent yet to the client, you don't get "IllegalStateException".
You can read more about 'Action jsp:forward' here regards,
MZ