Printing presses whether they're an older manual model (like the one pictured) or a modern computerized system, share one purpose in common - they must be able to lay down a film of ink with remarkable precision and consistency.
For a typical offset press, one ink tower delivering one of the four primary colors in full color printing lays down an image covering an area approximately 40" x 28". That image is formed by splitting a film of ink 4 microns thick (a tenth the thickness of a human hair) twice (plate to blanket then blanket to paper), while at the same time emulsifying it in a chemical solution (made primarily of water) to a depth of a few molecules. Too much water and the ink washes away. Not enough water and ink starts to print in the background. The ink is carried by approximately 19,250,000 halftone dots averaging in size from 10 to 60 microns. The final film of ink deposited on paper over the whole 1,120 square inch area of the image and is held to a thickness of about 1 micron (one fortieth the thickness of a human hair) with a tolerance of +/- one-tenth of a micron. The positional accuracy of the image is held within one-three hundredths of an inch - about 40 microns. All in one quarter of a second on press.