As far as I know, the oesophagus in vertebrates has no digestive or absorptive function. It is simply a conduit from the mouth to more distal parts of the gastrointestinal tract. Why have it at all, then? What evolutionary advantage does it give? Why not have the stomach start right where the mouth/pharynx ends?
Could it be to make space for the heart?
A Google search did not yield any useful answers.