The Greeks, for example, had multiple deities because they assumed that because we were human, that they were also human but with supernatural powers. That means they also had human frailties like jealousy. rage and also had sexual desires and the desire to kill. Having multiple deities was, kind of like going to a supermarket. If you didn't like one deity you might chose another. For example if I want a girl to fall for me I could ask the love deity or the god of jealousy or the god of deception and mischief.