The color adjustment of the printing device is the last obstacle between the photographer or the graphics designer and the viewers who are supposed to appreciate your work. Unlike the other stages of color management (camera profiling and monitor calibration) most people intuitively know one way to make their prints match what they see on screen. They print a test photo, make some adjustments on the computer, then print another test photo, make some more adjustments... expensive ink and art papers go to waste; frustration builds up and time is irreversibly
The devices that generate color profiles for printers are called spectrophotometers. Their job is to save you a lot of expenses running all these costly and time-consuming test prints. The color profile they generate has two uses. (1) While editing your image on-screen, you can load the printer profile in your image processing program and evaluate the effect of inks and paper on the contrast and colors of your image; you can then compensate until you get the desired result. This process is called soft-proofing. (2) At print time, the color profile is again loaded into the image processing software. It takes over the control of the printer and transfers to paper what you are seeing on your screen.
If you ask an experienced print professional, he will tell you that it is not always possible to achieve 100% match between what you see on screen and what appears on paper. This is understandable, considering the fundamental difference in operation principles between displays (which emit light) and prints (which reflect light). That is why using printer color profiles is so important when you try to bridge the gap between screen and paper.