Therapists guide people on a path of introspection and self-awareness....which is something most people could do by themselves, but if they're too far in denial they might need help. Some people spend their whole lives lying to themselves to the point where they don't even know how to be honest internally.
A therapist can't help you any more than you can already help yourself, because they're just going to ask you questions that prompt you to self-evaluate your life. Also, if you lie to the therapist it'll be the same as lying to yourself. They won't be able to help at all.
If someone is going off the deep end or having thoughts of self harm they should immediately be guided to a professional. For other people it might be helpful to take up journaling or something, and see what a little bit of daily self-reflection can change as a precursor to therapy.