A clipped bird is more dependent on its owner and thus can be more easily taught to trust him or her. Many times flighted birds develop attitude problems as they challenge their owner for the position of flock leader and become unhandlable. Some even begin biting. Third, you may clip to prevent escapes. This is not foolproof but it can help.
Cockatiels are light-bodied birds who fly with great ease.
For either Dandi or Rocky it only takes 2 flights on each wing half grown out to be able to fly. Pearl, a particularly large and athletic hen, can fly with all primary flight feathers clipped and isn't grounded until she has 2 secondaries clipped as well. This is why I consider the popular method of clipping but leaving the outer 2 flights intact a waste of time and effort for a cockatiel. You can consider it a nearly sure thing that if you leave the bird unclipped and escape is possible it will occur. There are some good reasons not to clip under certain circumstances.