Cost is often given as a reason not to mediate, and there is the cost of the mediator and legal representatives if the parties choose to have them.
That cost is almost always less than the cost of proceeding to a trial, so if the mediation results in a settlement, there’s a cost saving. If the case doesn’t settle at mediation, it often does shortly afterwards, because the minds of the parties are very focused on how strong their case is, and what it would take to settle. You might think that they would know this anyway, but without mediation, cases can sometimes drift along for months or years, and only as a trial approaches do the parties seriously consider their positions and what they hope to achieve.