You will write a lot of essays. Like it or not, if you're a student, you're going to write a lot of essays. Between homework assignments, the SAT and ACT, and college admission forms, your typing fingers are going to get a workout! Might as well learn how to get the job done better and quicker.
You'll learn to organize your thoughts. One of the hardest things for new writers is finding a logical flow for all of the creative thoughts they have running loose in their brains. An essay provides a format to corral those thoughts into something other people can read and understand.
Essays teach the basics of good writing. Essays contain all of the components and skills of quality writing. Once you learn those skills, you can apply them to any other type of writing. Thus, your writing life just got a whole lot easier!