The phrase in the title does not, unfortunately for most of us, refer to a butler who brings coffee to your desk in the morning. It refers to a computer in the office that has a few special tasks to perform on behalf of others. So, perhaps in a different way it's a little like a butler.
You could store all your files, emails, photos and more on your own computer. You could use only your computer for access to the Internet, printing, faxing and more. But that setup has some potential downsides, especially when your office grows to hold multiple desktops.