What is the purpose of cross- examination?

To ask for clarification, to expose weaknesses in the argument or evidence, to discredit the witness, or to elicit facts favorable to the party the cross-examiner is representing.

"Indeed, cross-examination is arguably the essential, if not sole, purpose of a criminal trial. Opening statements, the importance of which so many lawyers underestimate, is the foundation of effective cross-examination. The opening is where the lawyer not only provides the jury with the defense's version of the facts, but details how he will cross-examine prosecution witnesses. If the lawyer wastes the invaluable opportunity afforded by the opening, leaves critical "facts" unchallenged, neglects to cite examples of the witness's duplicity, and fails to tell the jury how he will cross-examine the witnesses, it's unlikely the jury will grasp the significance of otherwise effective cross-examination."