Beware! Over-simplification ahead!
Obviously, printed matter existed long before video monitors did. When televisions and video monitors were invented, they were only capable of displaying a grayscale, with a variety of intensities from total black on one end to total white on the other. Everything in-between was a variation on gray, with more or less white or more or less black blended to produce an image.
Color TV became widely available in the mid-1960s, and by the 1980s color computer monitors were on the scene. These color monitors used elements called phosphors to emit shades of color and reproduce what the human eye could see. Engineers wanted to produce the wide spectrum of color that was being displayed in the print shops. Since video screens emitted light, an RGB model had to be used.
The RGB color model is an additive color model in which red, green and blue light are added together in various ways to reproduce a broad array of colors. To create and display a color with RGB, three light beams (one red, one green, and one blue) are stacked. The additive RGB color model combines the three light beams and their wavelengths are added together to display color and artificially show what the eye sees.
The International Consortium of Color (ICC) created a standard to measure color monitors by and it was globally adapted for manufacturers.