Everyone's favorite nemesis, the check engine light.  It can mean so many different things ranging from a harmlessly loose fuel cap to a defective catalytic converter.  In any case, it still signifies a fault somewhere in the vehicle that will only get worse (read as cost more) the longer you ignore it.

True, you could run to your favorite parts store and they will scan it and give you a code number and maybe a list of possible causes.  Keep in mind they sell parts.  The more they can sell...

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Ok so now what?  Error codes don't really tell anyone what has definitively failed, but offers insight into which system may have a fault.  A fault code could be the result of a different failure altogether.  This is where the other sensor data can be so important in determining the actual source of the failure thus providing a successful repair.

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There are some common code faults that 9 out of 10 times are the actual problem, but we're all paying good money for facts, not guesses.