What happens if you ask your family what they think of your art work? "It looks really good," is the reaction you'll probably get. Not helpful, right. But with an art critique group, you hear what you need to hear. "What planet is that child from? Because I don't think that human children have arms that can do that." Maybe a bit harsh, but you need to know if your anatomical proportions are off. Other artists will tell you where to make changes and even suggest things that can be done to improve your work. If you are a newbie like me, those suggestions may be things you never even thought of. It may take a while, but after a few critiques you may find yourself learning to spot things that will help out your fellow illustrators as well.
An important thing about critiquing is that you learn to take criticism as a series of helpful comments that are intended to improve your craft. Don't go home feeling like you'll never get any better, leave knowing that you have just seen through someone else's eyes and you have a chance to make changes that will help you to improve.