With a calibrated monitor, you will know that when you change a colour on the monitor, you are changing it to exactly the same colour in the image file. You will know that the resultant image is what you want, and that the image on your monitor will match the print from your printer. If you're going to send the image to anyone else, or if you're displaying it on the web, then you can also be certain that anyone looking at it will see what you see (providing their monitors are calibrated too!). This is very important if you're sending the image to a printing company or a publishing house, as your neutral print on your uncalibrated monitor might well result in an image with an obvious colour cast for everyone else.

Monitor calibration is not a one-off operation either. It should be done regularly, at least once a month and more often if colour accuracy is important to you. The colour accuracy of monitors changes over time, so you have to keep on calibrating.