"under one banner by worshipping one god instead of different gods in different cities."

* most likely for administrative control and centralization. people have their own experience - people's spiritual lives have a lot to do with their locale. to try to usurp this reality into something else for reasons of control should be viewed with suspicion. monotheism perhaps could be summed up as equalling socio-political-economic need for centralization. religion does not exist in a vaccum - it is subject to the whims of the needs and will of the society and its leaders.