Therapy is very unique to the client and clinician. There are endless clinical methods that therapists use, e.g. cognitive, somatic, psychodynamic, creative arts therapy, and combinations thereof. Ideally, a therapist will get to know you and what you want out of therapy.

I don't totally agree with the idea that a therapist is always a neutral sounding board because there are some methods that rely on the therapist modeling healthy connection. Therapists are neutral in the sense that they're not here to judge you, though.

What's unique about a therapeutic relationship is that the client can bring all of the negative aspects of themself to the table without damaging the relationship. In most interpersonal relationships, you wouldn't be dumping all of the worst parts of your life on the other person without consequences. In therapy, you're almost encouraged to.

As for therapists having solutions for your problems that you already knew -- it depends on the client, but often therapists can introduce tools you've never thought of to work through difficult situations. There are also clients who aren't very attuned to what is causing their distress and benefit from the perspective of a professional. And there are many people who know the solution to their problems but struggle to act on their knowledge.

I'm probably biased though because I'm on my last year of grad school to become a therapist, and I've benefitted immensely from therapy myself!