A deposition gives both parties the opportunity gather evidence and background on the case, witnesses and to solidify the stories being told by the witnesses. This helps the lawyers on each side better prepare for a trial.
Videotaping or transcribing a deposition gives defendants and prosecutors the opportunity to review the facts in a documented, irrefutable form.
If a witness contradicts his deposition while testifying at trial, an attorney can impeach the witness' testimony by casting doubt on his credibility and consistency.
If a witness dies or is otherwise unavailable to testify at trial, a deposition can be shared with the jury in place of live testimony.
A deposition can be a determining factor in whether lawyers will decide to take the case before a jury or settle. During a deposition, witnesses are tested under pressure and attorneys can examine whether a jury will find each witness to be believable, honest or sympathetic.