One aspect of vipassana is to experience the fact of impermanence, the fact of change. And when we are aware of the breath, we’ll realise how whatever is happening in our mind and body, including the breathing, changes from moment to moment. So if you are having thoughts you’ll immediately realise how thoughts are arising, how thoughts are passing away. And the same thing becomes very clear with our breath.
Here again, if we can be aware of the sensations we’ll realise how from moment to moment there are varieties of sensations taking place, and again how sensations are changing from moment to moment. So we learn to be open to any changes that are taking place in our mind and body from moment to moment.