Calibrating a monitor ensures that colors are displayed accurately, as well as consistently across every display, even when printed.
Why Do Monitors Need Calibrating?
A monitor uses the RGB color space, where each pixel combines a certain amount of red, green and blue to output a specific color.
RGB values for each pixel are contained within an image (or video, or any other graphical element). On displays that have been properly calibrated, the image will always look the same.