The short answer is to guarantee that the right person is convicted and punished and that an innocent person is not. Proving a negative can be very difficult for those accused. Suppose that a crime was committed a few blocks from your home at 3:30 AM on January 14, 2009. Could you prove where you were then? The prosecutor must prove that you were there. State and federal bills of rights, legal codes, and rules of court procedure are designed to produce a correct and just verdict, but the judicial system does not always work flawlessly.