Breath is an important part of many religions.
Of course in the Old Testament, breath is what animates, or gives life to, a being.
In Eastern religions, breath has a unique significance. As a focus for spiritual practices, it overlaps with awareness in an interesting way.
Awareness is considered the root of spirituality, just as breath is considered the root of "life" (i.e., the life in a body). So awareness of breath is as essential an awareness as you can get.
But there is a more important aspect of breath. As a function in humans, it can be either conscious or unconscious. In other words, breath exists as one of the most basic functions. Whether you are awake or asleep, you are breathing. But awareness is fleeting. Still, awareness does not exist in the unconscious (or at least it is not noticeable).
So, combining awareness and breath causes breath to become a sort of threshold between consciousness and unconsciousness, or the subconscious. In other words, or in the language of Jung, awareness of breath allows access to the most primitive forms of elements that had been sacrificed to form the ego.