Say you have Schizophrenia, you will meet a psychologist, he will evaluate you. He will instantly refer you to a psychiatrist, because he is the one to solve your condition. Psychotherapist can solve psychological problems, sometime without medication. But they cannot intervene on mental disorder itself. That is why you have both, because having a mental disorder usually come with a lot of psychological problems along side with it, even once the psychiatrist threatement. Psychotherapist is then the one to help on these issues.