This summary is not the rule. The rule text controls.
If someone has important information for a criminal case but might not be able to make it to trial, either side can ask a judge to let that person give their testimony early. This usually happens when a witness is about to move out of state, or when they're too sick or weak to guarantee they'll still be around or able to show up when the trial actually happens.
Both the defense and the prosecution have the right to make this request. The goal is simple: if there's a real risk that a witness won't be available later, the court can arrange for their testimony to be taken ahead of time. This way, their statements can still be used as evidence even if they can't physically attend the trial.
This protects both sides. Prosecutors don't lose a key witness's testimony just because that person got sick or moved away. Defendants also benefit, since they get the chance to question the witness and challenge their statements before the trial starts, rather than having no opportunity to respond to their claims at all.