Hair color isn't always about just "color". It's about shades, hues, and a good cut that will show off your new color. Perfect color doesn't come in a bottle. If you are hoping to get a color correction for past mistakes (caused either by you or another salon), it will take a real expert to add in the right toners to counter the faulty shades you are trying to get rid of.
Plus few things offer the opportunity to change your look the way changing the shade of your tresses does. It lets you keep everything else virtually the same while still giving you the change you seek. Not a bad deal actually.