Let's say you are writing a webserver. Your webserver has received an HTTP request. You have read it in, decided on what page to return in response, and are now piecing the page together out of pieces of data and HTML. After each bit of HTML has been created you write it to the socket with send(). At some point, you write the final "</html>" tag. At this point you have finished writing the response, and you want the computer to send it back to the client ASAP.
However, since there is no flush method in the sockets API, you can't tell it to send immediately. Instead the sockets implementation just dicks around for say 0.2 seconds (or whatever the implementation timeout value is) before it decides to finally send the last packet. This seems quite silly to me. There should be some kind of flushHint() API call that would tell the implementation to send the last packet ASAP.