Although a default display profile is assigned to your monitor by your operating system, you'll get the best results from a profile customized for your specific monitor. Photoshop gets this information from the display profile. To display color accurately, Photoshop needs to know how your monitor behaves-what color white it produces, what sort of tonal response it has, and what actual colors it produces when it's fed pure R, G, or B. To make this magic happen for you, you need an accurate profile for each monitor.
This makes monitor calibration a mission-critical necessity! (The only time this behavior doesn't happen is if you set the working space to Monitor RGB, which we don't recommend.) The great benefit of this approach is that it makes it possible for people using very different monitors on different platforms to view the same image as consistently as possible. Photoshop uses the monitor's profile to transform color data on the fly as it gets sent to the video card so that the monitor displays the color correctly. If your monitor profile doesn't accurately describe the actual behavior of your monitor, your judgments about your images won't be accurate, which means your corrections will also be inaccurate.
Photoshop displays everything through your monitor profile. Learn More Buy Photoshop and Your Monitor Real World Adobe Photoshop CS4 for Photographers
