What is the advantages of cross examination?

Cross examination provides the opportunity challenge "direct testimony" given by a witness. It is a fact finding mission. It is a way to expand on the testimony and get to "the rest of the story". You can't assume that witness testimony is without bias, is full on omissions or is just plain perjury. Within the legal scope of cross examination, you can ask questions that force the witness to reveal more information that can bring a new light to the testimony and its context.