Well, for starters, preparing an outline for an essay test usually helps you think through what you want to cover when writing. It's almost exactly like starting with a stick figure when drawing.
Also, when you plan out what you want to write, it'll be easier, and faster to write. Not only that, but your point will be more clear to your audience if you plan out your essay, versus writing what comes to mind.
If you have a better essay, you're more than likely going to achieve a better grade.
