Can you use a server as a PC?

YES, you can. Though its not recommended. Actually its highly recommended you dont use it as a PC. But if you insist on doing it you can.

The problem is people have a tendency to mess up PC's or download viruses onto it. Or clutter it up with junk, start messing around with settings they dont understand or install a program which interferes with the servers day to day duties. So for this reason, most network admins, tend to set-up the server and get it all working and then god help anybody who so much as moves the mouse on it.

Yes, in fact a server IS a PC. Server just describes the fact that it is capable of being a file server or other network server for other networked computers.