I just read this article and his last paragraph stood out to me, in which he claimed that what helped most in therapy wasn't the "work" we typically do (coping, dealing with the past), but, in his words, was the "look that conveyed a faith in me that I did not feel at the time." This resounded inside me. Sometimes I wonder why I still go to therapy. I know lots of things to do to help with my depression and anxiety. But I want to see a person that knows what I'm dealing with and is encouraging me to keep going. Sure, I've heard the criticism that we can get a listening ear from a good friend, so why do we have to pay someone to be a friend. And I do have the "Doubting Disease," after all, so it shouldn't surprise anyone that I wonder if I should be "paying" someone to listen and support me. But I do pay someone to listen to me and support me and help me keep going in the face of depression and anxiety. I have friends, too. But my friendships work along with my therapist in helping me deal with life. It was encouraging to see that I'm not the only one helped by therapy in this way.
I also think that having a therapist takes some of the pressure off of my friends. My therapist and psychiatrist can worry about making the call regarding how serious my depression is. My friends may sometimes help in that, but they don't have to feel responsible for me since "professionals" are involved. I know it helps me feel less pressure if someone sees "professionals." And taking off that bit of pressure gives me freedom to be more of a friend with less feeling of need to protect myself at their expense. Parts of their care that I can't handle (and am not trained to handle) can be taken care of by their therapist or psychiatrist, and I can support them just as a friend.
So maybe that's my answer to the "why do people pay for a friend" arguement. I have friends, and I have a counselor, and together, they help me, each in their own way, each better able to because of the other person fulfilling their own role.