I think it can certainly help a fledgling relationship, but it's not going to save every relationship, in fact it may help someone realize the relationship isn't right for them and could result in that relationship breaking off.
I think it depends on each couple and how their connection is. Because you can't save every relationship, sometimes there's no saving a bad one, and the best thing you can do is break it off and move on. But, I'm not saying every relationship will go this way. Some will find success with therapy, while some won't.
Sometimes that significant other isn't the right person for you, and if it takes therapy to realize that, than that's why therapy is so good. It helps you get to the bottom of it, and hopefully fix something or change it. Does that make sense?