By calibrating using software and colorimeter, you are calibrating to a known standard. This helps insure that colors, photos etc. will reproduce correctly on other monitors, printers etc.
This may not be important to anyone where precise colors aren't critical. But to photographers, videographers, web designers etc. it is of supreme importance. e.g. If you are a professional photographer doing model shoots and your uncalibrated monitor is a bit on the blueish side, all those photos that looked great when doing photoshop touchups will look a bit sickly yellow to everyone else. Not good.
I should get my 27" iMac in a few days. One of the very first things I will do is calibrating the monitor. It is a fast and painless process, and I have yet to see any monitor that wasn't improved with calibration. Once done, it is important to periodically repeat the process as monitors change over time and usage.