Hypnotherapy is used in order to by pass the concious mind - the bit that sees hears tastes and feels and feeds info back to the subconscious for evaluation and learning.
We have a concious mind - the bit take info in and the unconscious the bit that stops that info so we don't have to keep learning it.
The sub conscious controls our actions and is driven by our belief systems. We actually spent 90% of our day in subconscious mode. For example - have you ever been driving and you arrive but don't remember the journey, or so engrossed in a book time just flies by? That's because once we learn to do something the subconscious just does it with out conscious thought.
All good until the subconscious learns a behaviour of a belief that is in some way holding you back, like fears or phobias, anxiety about something or depression.
Often there is no conscious reason for these debilitating beliefs and behaviours - you are actually in a trance - i.e. a behaviour you have no control over.