@CousinTed: Because God is supposed to be wise, just, and good. He's supposed to be our Father. So, if that is the case, then one eventually wonders why is there so much evil and suffering in the world. The Catholics can only emphasize original sin up to the point where people start to wonder, "Yeah, but where did this original sin come from, anyway? Why is it there to begin with?"
And what about all of those other Christian denominations who don't believe in original sin? Again, they too can only point out to mankind's flaws and failings for so long before someone inevitably asks, "Well, just why are people so bad? What makes them that way? What causes people as real as you or me to do such terrible, awful things?" The explanation "people are bad because they're bad and that's that" may suffice for small children, but isn't very convincing or reassuring for adults.
Ergo, another figure, another source is needed in order to supply a simple explanation for a complex problem. God is benevolent, but Satan is bad. Satan caused original sin by tricking Eve into eating the fruit from the Tree of Knowledge. Satan is a wily tempter who tries to sway mankind from the path of righteousness. He either convinces humans to do the wrong thing or possesses them. Satan needs to be the scapegoat just so God doesn't have to bear the brunt of blame for anything. Christians can be secure in their belief of a loving, benevolent God while simultaneously having an explanation for evil.
It's monotheistic in the sense that Christians don't pray to Satan or worship him; they only worship God. They believe in Satan, but they don't believe that he is a god, just a lesser demonic being who is nevertheless powerful enough to cause all of the suffering in the world.