Impeaching a witness at trial (or in deposition) through cross-examination damages the credibility and trustworthiness of the witness
Impeaching a witness is fundamental to trial advocacy. The goal is to weaken or seriously damage the credibility of a testifying witness.
Impeachment is when the lawyer cross-examines the witness in such a way as to reveal the witness has lied under oath, is biased against one of the parties, or has made prior inconsistent statements under oath during the discovery process.