[dredmorbius]

Because very often:

1. It's not about the disagreement. It's about what's behind the disagreement.

2. Many people cannot or will not discuss matters amongst themselves. Getting an impartial third-party engaged (and one who doesn't have a stake in future relationship(s) with the disputants) can have a hugely useful "unsticking" function.

Mind, there are many therapists and therapy methods, not all work, and some practitioners are incredibly ineffective.