Why do authors use monsters in their stories?

I think you are on the right track with your idea. A children's author would use monsters to teach a child a story. Now you just have to think about why adult authors use monsters. Take the Lord of the Rings, as an example. That book can actually have deep political meanings if you look at how the monsters in those stories represent how absolute power can corrupt a person/ruler. Now, the fact that monsters are used in that story is because it is much more fun to imagine giant eyes and cave trolls than it is to imagine men in suits and ties. So again, you are on the right track.