If you print or sell your photography as part of a professional workflow, it is very important that all of the computers that you use for post processing have been properly calibrated. And in addition to calibration, each should be set for the same color space.
Your work environment is equally important. It should consist of a light that is slightly subdued, neutral in color, and does not vary throughout the day. Strong light or bright colors in your field of vision will likely influence your perception of contrast and color, so it pays to plan your working space to ensure it is comfortable for your eyes with no reflections, or deep shadows.
Why does the calibration of your monitor matter? The goal is to ensure standardization between what you see on your screen and what the printed photograph will look like whether you print it yourself or have a professional lab do it. Pro labs will have their machines properly calibrated, and if you do not, your cherry red could print out looking more like fuscia purple.
Colorimeters or calibration devices will properly measure ambient room light, as well as gamma, contrast, black point, white point, and properly adjust the colors and temperature of your monitor. A profile is then saved which you can use in your editing software.
I use datacolor’s Spyder3Elite color calibration device, and have been very happy with the results.