You upload a 100 KB PHP/Python/Ruby/whatever file to your webserver.
During the upload (especially on slow lines) the file can possibly be retrieved partially because parsing doesn't work yet.
Just imagine the login data to your database can be seen in clear text in the browser of someone accidentally loading a website while you upload it.
This also affects editing and saving a file in WinSCP.
Option for uploading the file renamed like $FILENAME_temp$threerandomchars and automatic renaming to the original filename when uploading is finished.