Scholarship committees sometimes get thousands of scholarship applications a year. A written essay can help the committee narrow the applicant pool by automatically discarding any applications that don't follow the essay requirements.
The essay allows you to show the committee that you can think deeply about the question it asks. The committee wants to award money to a student who can understand and question the information he or she learns at college. You can show you have the skills to do this by answering the essay question well.
A scholarship committee wants to know you can use grammar and sentence syntax appropriately. This allows it to assess how well you'll succeed in the classroom, ensuring that the money it gives you won't be wasted.
The essay is your chance to be creative. You can stand out from the rest of the applicant pool by writing your essay in an intelligent way that other applicants wouldn't have thought about doing.
The essay allows the committee to know a little about you. Even if it isn't a personal essay, they way you write and the way you answer the essay prompt says something about your style and philosophy on life. When a committee gives out money, it wants to know what kind of person the money will go to.