Psychotherapy is a bit like nuclear engineering. Everyone has events in their past that they would rather forget. For some people these events are intolerably unpleasant. They develop coping strategies.
Putting your psychic waste in a little black box is very much like storing nuclear waste in a cooling pond. You know there'll be a leak, oh, say, every ten thousand years on average. You just hope that it won't happen until you can cope with it.
Psychotherapy is where you employ a shrink to replicate the role of an AEC inspector. They wander through your brain, engaging you in conversation, looking for reservoirs of hot liquor; when they find some, they kick at the barrel to see if it rumbles.
I'm not sure what they're supposed to achieve so much as I am sure that therapy, especially the early stages, are about digging up some really, really painful stuff; stuff like the time in school when you blurted out the wrong thing in front of the entire school; the time when your father tried to kill you; the time when you got made homeless because you were scared and alone.
Psychotherapy is a process of discovery; by working out what you have in your head, what there is stored away, it can't make you crash when it turns up. Therapists can help you discover what there is stored in your head and help you deal with it in non-destructive ways.
In the early stages, this does not help.