First, to reduce the color deviation on neutral colors (grayscale)
The imaging method used in the printer is CMYK color system (C-cyan, cyan; M-magenta, magenta; Y-yellow, yellow; K-key, key color), CMYK color system belongs to subtractive color system, that is, different colors The more superimposed, the darker the color, and the final CMY will be synthesized into black.
But with current technology, CMY cannot be synthesized into pure black, and the final synthesized color is dark gray or dark brown. Therefore, when using color ink to synthesize black, you need to add black ink to make the color more pure. In the same way, when you want to print a dark color, in addition to adding a different proportion of color ink, you also need to add a certain proportion of black ink to make the color density higher and more pure.

Second, this is the result of both quality and cost
In the actual printing process, black is used the most and most frequently. If CMY is used to synthesize black, the speed will be slower, the consumption will be more, and the cost will be relatively high. Using black ink to print black documents only consumes black ink, which is more economical and cost-effective, and the black will be more pure.

Finally, it is to increase the density and density of the printed document
In some printing, it is necessary to use color ink to synthesize black. For example, when printing photos, some black inks are pigment inks, which are not compatible with photo paper. Therefore, it is necessary to use color inks to synthesize black, or use special photo black ink to print photos.