Involving a third object party who brings wisdom into the situation will always help; this assumes a good counselor. I have known many good, even gifted counselors, and I have known many terrible ones who frankly made the situation much worse.
So, getting the right counselor is critical. However, a good counselor will help create and build bridges that the couple can use to overcome what seems insurmountable. And I guess it depends on bad circumstances; a bad circumstance can be the death of a child. This can take a very heavy toll on the parents, and oftentimes, marriages end up shipwrecked. A counselor can help them constructively navigate this and help the couple learn to serve and help one another in a positive direction.
In cases of abuse, the abused rarely perceives it as such, and a counselor can add light to that and work to build boundaries and safety for the abused. In the end, a counselor is a lot cheaper than a divorce in practical terms, and in my experience, most (not all, abuse, adultery, abandonment, etc.) divorces could have been avoided early on, but that fixable foot to the left or to the right ends up being miles away by the time they are involved.