Network administrators typically need to interact with many servers in an enterprise environment. There are hardware devices called KVM switches. A KVM (keyboard video mouse) switch connects to a keyboard, a mouse and a monitor on one side, and it connects to the keyboard ports, mouse ports and video ports of a number of servers (or desktops).
KVM switches work well, but they typically have a limited range because video signals cannot transmit properly over long cables. VNC is a protocol and mechanism to remotely interact with GUI environments. This makes it possible to use the GUI environments of computers that are network connected to an administrators own machine. Note that although VNC works better in a LAN with plenty of network bandwidth, it does work sufficiently over the Internet with somewhat limited bandwidth.