Why is percent error important?

Percent error tells you how badly things went wrong.

Here's the deal: Whenever you do an experiment, things go wrong - that's true for anybody no matter how good they are. Things get spilled, things are impure, equipment is imprecise... you get the idea. No matter who you are or how long you've been doing science, this will always be the case.

A percent error calculation simply tells you the magnitude of the mistakes that took place during the experiment. If you have a 5% error, then you've done pretty well. A 95% error suggests that you might want to try something new.

Simple and compares similar terms here: http://www.mathsisfun.com/data/percentage-difference-vs-error.html