It gives you the chance to show your enthusiasm for the position at hand, to discuss how the skills the position requires fit into your ultimate
career goal and to explain a
bit about the environment in which you'd ultimately like to work.
Write
about the job itself and your qualifications but then impress the reader with a little
bit of flattery, for instance, «After reading several other job ads and researching your company, I felt EMC was the best for my future
career goals.»