Comparison/contrast essays help you learn really efficiently, because they present information in a way our brains like. When we see something new, we want to know how it's similar to something we already know about, and how it's different, too.
Let's say that you need to buy a car. When you look at the different models, you'll compare what's similar and different between the models. When a cell phone manufacturer comes up with a new model, they tell you what great features the phone has that people have liked before (the similarities) and what fantastic new features it has that help it stand out (the differences).