A normally functioning body "cleanses" itself. It doesn't require any outside intervention to do so. There is no study or evidence that colon cleansing helps with long-term weight loss or disease prevention in normal bodies. Your body has whole systems devoted to cleansing any toxins you encounter out, including your liver, kidneys, and colon. It doesn't need pills to do so.
If your body is NOT normally functioning, you'd be better served working with a doctor to correct whatever is going on. And everyone (except those with certain medical conditions) would benefit from getting plenty of fiber and water to keep things "moving". Beyond that, though? No benefits have ever been shown.
There's a great article on this at Quackwatch called "
Gastrointestinal Quackery"