why should you disclose your behavior in anger management?

Answer 1

Because it is more instructive and safe to do so : shed your anger, resentment, etc., in a controlled environment than out in the street or among friends family, spouse, children or strangers.
Someone could be hurt out there, who has nothing to do with your uncontrolled feelings of wrath.

Answer 2

If you are in anger management classes, then you might have done something (by your behavior) that shows you are out of control. Behavior and anger management go hand in hand. You need to be truthful here or you are not helping yourself out.

Answer 3

Once you address an issue, it is easier to make the needed changes.