There are one of two distinct possibilities for why this region of the brain could exist.
- The faculty for religious belief was built into us by a creator.
- Religious belief, or spirituality, was a trait which led to a greater surivival rate of those possessing it, likely in pre-homo sapien man, resulting in it being present in all homo sapiens sapiens.
These two can basically be summed up as such: "God created man, or man created God."
Currently, the vast load of empirical scientific evidence points towards the latter being true, since evolution is currently the only theory which has stood up to decades of scientific scrutinizing.