Real weightlifting and powerlifting belts are usually 10mm and 13mm but as mentioned not necessarily set in stone. My wife prefers her 8mm. The reason for the thickness of the belt is not to actually train the abdominals but rather to give something rigid for your core to press against so that you can build pressure throughout your mid-section (Vasalva Maneuver). When I'm squatting and deadlifting, especially heavy, it is essential to remain extremely tight during the course of the lift and that's where the belt helps. Before I begin my lift, I take in a large gulp of air and concentrate on expanding my core the whole circumference of the belt.