Mystics and priests, who would have otherwise led bland lives, suddenly saw the power to be had by placing themselves as the middle-men between powerful gods and the lowly population of humans. They could influence governments and large populations without even having the backing of an army. If they didn't know the answer to a particularly difficult problem, then it could just be explained away as divine glory that we weren't meant to understand.
Every culture has had people who wanted to exert control over their society. Some did it with military might, others with economic might, but there was always the poor-man's method of control: religion. Even to this day we can find those who "answer the call" placing themselves up on a pedestal in front of a captive audience.
