The U.S. Constitution guarantees people charged with crimes will have the write to 'cross-examine their accusers.' In effect, the defendant or his attorney has the right to ask victims about the reported crime, including questions about what happened, how it happened, when it happened, and other details.
A written statement is only that-a written statement. There is no opportunity for the defendant to question the victim about the case. Therefore, the complaining witness in a case (also known as the victim) will always be needed to appear in court to testify, unless there are special circumstances, such as a plea agreement.