This is actually a two-part question, but many people miss that!
PART 1: Demonstrate that you understand what the job and company are all about! This means more than simply regurgitating the job description. Find a way to show that you have been to the website, and truly know the company's business.
PART 2: Why you are a good fit for the job. How will you be an asset to the company? That is, How will you help them to make or save money (the ultimate objective of all businesses!) Use whatever you have! "Having recently graduated from HS with a rigorous courseload, I am accustomed to learning complex new material quickly. This is important at XYZ corporation, as new product lines are introduced monthly." One that I used, to demonstrate that I knew a security position was not looking for a gung-ho cop "I recently retired from a rewarding career as a police officer at the San Antonio Airport, a specialized policing environment with a dual emphasis on both customer satisfaction and public safety. This experience has prepared me well for the unique role of Security and Public Safety Officer with your Museum."
As you can see, you can address both part 1 and part 2 in the same sentence. The important thing to remember is to address both parts!
Let me know if you have any questions!