Guilt is a useless emotion...but a powerful one. And people who know how will use it to control how others act or feel.
What is reallly insidous about his emotional tool is that it is not used by people we are already wary of, don't trust or don't like, but the very people that we already know, trust, rely on. Parents, siblings, friends, religion.
Many mothers (and some fathers) use guilt to attempt to control the thoughts, feelings and actions of their grown children. It's so common that many jokes are made about it. It is the standard comic routine of many a Jewish comedian who had an overbearing, guilt-inducing mother.
Even friends use guilt to get control their friends actions and feelings toward themselves. Afraid of losing them as friends or just some need to control.
That's how advice columnists make a living. Giving two minute lessons in how to handle manipulative, guilt-inducing friends and family.
Religion is one of the biggest abusers of this emotion. They guilt their adherents in to proving their love for a deity in the amount of money, time and resources they give to the church.
Of the religions that use guilt, cult like religions, such as the JWs, use it ruthlessly. Nothing you do is ever good enough. And you very salvation is dependant on your works. So if you have to prove your worthy of salvation and are constanly reminded to do more, the guilt builds and builds, no matter how much you give.
Once you realize the power of how this tool works and the kind of people and organizations that use it, you find yourself more resistant to it's effectivness. People no longer can 'guilt' you into doing something you don't want to do. Friends and family no longer drive you crazy with their mindgames and manipulation. You see what they're up to and you take measures to avoid those situations...or them, if necessary. You see through religions and other groups that want to use you to their own end.
Living a guilt free life is one important way to living a happy, satisfying life.