Why do doctors wear white and lawyers black?
four color dots in the newspaper called CMYK (cyan, magenta, yellow and black). These base colors are used in printing. Any color can be obtained by using correct proportions. The marker is also used to identify the density of colors and dot formation.
You may not know the exact number of newspapers that are printed daily, but it common that they are done so in abundance and manually checking all the pages of the paper is not possible.
For a printer, who has been doing this for years, knows what an apt CMYK(cyan, magenta, yellow and black) looks like. If there is anything off, he will be able to spot it. So basically they serves as 'printer's marker'.