Absolutely essential. You can get things like Spyder Pro which will do it for you.
If you're going to be doing colour printing - then you will need to be able to get the CMYK, RGB, HEX swatches from the Pantone Books - as a reference.
Plus you don't want to be giving clients non-calibrated printed proofs to sign off on - they will get the printed product back and it will have shifted in the colour - citing good reasons for a reprint or a their money back.
You may also need to colour match a textile swatch, or other swatch a client brings to you - like a cutout from a magazine or other.
On another note - and not to be a grammar freak - but you have some serious misspelling issues in your post confusing, there, their and they're.
I think it's best practice to always be on top of your grammar, forums, emails, or just general correspondence with someone.