Religion gives those who follow it an outline for how to live, how to act, justice, good and evil, and how to treat others. Without religion humans will always revert to a survival of the fittest mindset. Ancient humans would develop religions to bring others to them who agree with those ideals. Think about it this way, if you wanted everyone in a town to show compassion to each other but aggression to outsiders which is more effective? Telling them this yourself, or saying a mighty and wise god told them to. If they accept the message then others will hear about it and if they share the same values they will join you. This is only one of many reasons for religion in a growing society but it is good enough for a short answer.
