Ubuntu Server is distributed with no GUI, so it can do its server tasks most
efficiently. However, if you want a system that can *test* server functions,
such as testing a web site, you can install all the server functions into Ubuntu
desktop. So, for example, my desktop system has Apache and Mysql running all the
time. Takes a little bit of memory, but I have lots.

Most people run real servers with no monitor or keyboard, and use SSH to control
them from another computer.