I have previously done some volunteer work as a student for somewhere that had opportunities for couples therapy and i they said some stuff that really changed my view of what it would be. Apologies if this is all well known
Firstly they said that their aim isnt to keep the couple together but rather to give the couple space to talk about what they need etc. So if you go and leave the relationship you havent "failed" but that it might be a success if you've realised the relationship wouldnt work for you anymore.
Equally that its not about blame and parties have to want to go foward together. So for example i see lots of recommendations for couples therapy on here when both parties clearly have had enough eg after infidelity. The therapist shouldnt just be blaming one party for causing the issues but rather working a way to move foward. This can feel really unfair if you are the "wronged" party. They cant act as a judge to say where the fault lies.
They often saw people separately at some point, and theres lots of times when simply having a space to talk without disruptions can be helpful. A good therapist should work on a way to let you both feel heard which can break cycles of communication (eg as someone said if one partner talks over, or cries or you both end up normally storming out).
In an abusive relationship it can leave you at more risk
I have watched 2 friends go through it, one it seems to have made a signficant difference. I think the other needed the therapist to give them permission to leave or was hoping that somehow her dp would change (when he already knew the issues but didnt care enough to change).