A deposition is sworn, recorded, out-of-court testimony taken under questioning by opposing counsel. It serves three purposes: learning what the witness knows, locking their account in place, and creating usable material for summary judgment and trial.
The second purpose is the one witnesses underestimate. A transcript is a permanent record; every later inconsistency will be put beside it. That is why preparation focuses less on knowing answers and more on discipline: listen to the question actually asked, answer only that, say you do not know when you do not, and do not speculate.
Corporate depositions are different in kind. A party may notice a company to testify on specified topics, and the company must designate and prepare someone to speak for it — whose testimony binds the company regardless of personal knowledge. Preparing that designee is one of the most consequential tasks in a commercial case, and inadequate preparation is itself sanctionable.