What is does a monitor need a pc?

A monitor absolutely needs a PC (or some other compatible device) to function and display an image. A monitor itself is essentially just a display screen; it doesn't have any processing power or the ability to generate images on its own. It relies on an external device, such as a:

...to send it video signals.

The PC (or other device) processes the information and generates the image, then sends that information to the monitor through a video cable (e.g., HDMI, DisplayPort, VGA). The monitor then simply displays the image it receives. Without a connected device sending signals, the monitor will typically show a "no signal" message or remain blank.