For the answer to this question, you need to look at the definition of the word therapy in the context of psychology.
Therapy is a treatment for psychological problems in which therapists and clients work together to understand problems and come up with plans for fixing them, generally by changing ineffective thoughts, emotions or behaviors (APA, n.d. [in the website meta description not in the web content]).
The borders between therapy and general socratic questioning between friends/work colleagues is very thin (if at all existent). It can be very therapeutic to feel you can talk to a very good friend or colleague about a problem, but sometimes the situation can be misjudged. With all the best will in the world, sometimes people find it difficult to be non-judgemental of a friend who has confided in them.
Trained therapists, who are registered and licenced, are vetted by their managing bodies to provide a safe therapeutic environment to their clients, free of judgement and respectful of client autonomy (Fundamental principles of all codes of ethics — e.g. National Counselling Society (updated June 2020)).
Therapy provides a supportive, non-judgemental environment where you might feel more able to talk openly about your experiences.
Your therapist and you will work together to make changes that you want to achieve to feel a greater sense of happiness, empowerment, or perhaps to feel less affected by particular experiences (Devon Partnership NHS Trust, n.d.).
You said that
in a therapeutic relationship the therapist has power (which can be used to exploit the client if they want)
Remember that although therapists are in a position of power, if true autonomy is provided, the client has all the power to control the therapeutic relationship.
Among other big differences including closed vs open questioning, one big difference between friends/colleagues and a therapist who works within the ethical framework is that a friend/colleague may tell you what they would do or suggest what you should do to deal with a situation.
Respecting client autonomy, a therapist can not and must not do that. An ethical therapist will work with the client to work out what options the client has and possible pros and cons associated with those options in order for the client to choose their own course of action.
Psychotherapy is a collaborative treatment based on the relationship between an individual and a psychologist. A psychologist provides a supportive environment that allows you to talk openly with someone who is objective, neutral and nonjudgmental (APA, n.d.).
APA. (n.d.) Therapy. American Psychological Association. https://www.apa.org/topics/therapy
Devon Partnership NHS Trust. (n.d.) What are psychological therapies? https://www.dpt.nhs.uk/our-services/psychology-and-psychological-therapies/what-are-psychological-therapies
National Counselling Society (2020). Code of Ethical Practice https://nationalcounsellingsociety.org/about-us/code-of-ethics