Often, it is preferable to remotely connect to and control Raspberry Pi across the network, for instance, using a laptop or desktop computer as a screen and keyboard, or while Raspberry Pi is connected elsewhere, perhaps even connected to some hardware it needs to be close to.

VNC is just one way in which you can remotely connect to Raspberry Pi. It will create a new desktop session that will be controlled and accessed remotely. The VNC session here is separate from the one that may be active on Raspberry Pi's display.