Calm is a factor of mind that balances the energy and rapture. Mindfulness and investigation will be weak if calm is not developed in conjunction with the other factors. Without calm, the energy asnd rapture will lead to a manic, aroused, and volatile state of mind, that will miss the subtler aspects of mind because they are overpowered by the strength of rapture and energy.
When the mind experiences the blissful pleasure of jhana and unification, it reaches a state of still, calm, tranquility. This is a peaceful quality of mind that is an element of mind that leads toward awakening. The calm peacefulness and equanimity is balanced with the energy and mindfulness- we have the penetrating resolution and also we don’t attach to objects – both elements are required in order to experience non-self, and that is why these are factors of awakening.