among many other things,
** creates a safe space where anger can be expressed, questions can be asked, things can be said out loud that can't be said anywhere else (remember the thing about we don't tell the T things so T will know what happened, we say things so that we may hear ourselves speaking the truth about our own lives);
** records the incidents, the insights, the dots, as it were, to show later how they connect. Which the individual may miss, or may forget, or gloss over, or even deny.
** Keeps things on track; doesn't allow getting stuck.
** in some therapies, helps the person to self-acceptance by modeling a non-judgmental reception of the facts (etc etc)
** by challenging the comfort zone, as another PC person admirably put it;
** In some therapies (such as trauma), provides a stable, trustworthy relationship to help the person re-learn a worldview in which there is such a thing as trustworthiness and stability. I;ve heard it said that damage done in relationship has to be healed in relationship.