There are many differences between psychotherapy and counselling, which at different times can be important to an individual seeking help in their lives. At a practical level psychotherapy is usually a longer process, not time limited as counselling or CBT often is.
Experientially, clients often find psychotherapy gives them a greater insight and understanding into their lives. It might be shedding light on what has made them the person they are today. Or maybe giving a voice to past events that on the face of it you would rather forget, but deep down you know it affects who you are today.
So part of the psychotherapeutic process is insight, but as important is finding a way to lead a life that is not shackled to the past.
Psychotherapy is helpful if you feel stuck, either personally, professionally or relationally. It's about helping resolve the big questions in your life; the kind of things that you find going round and round in your head as you lie awake at night.
Psychotherapy often follows on from an initial course of counselling. Maybe you've resolved some current problems in your life and learnt some techniques to cope better. But your're still left with the nagging feeling that your not really getting to the root of your unhappiness.