Body Language is what your client is telling you before they speak
Body Language is important for hypnotherapy. When I meet a client for the first time, I hand them my clipboard. I ask them to write down their name and email address. And while they are doing that I inspect them minutely and try to figure out what their body language is telling me. I look at their fingernails, bitten nails are a sign of chronic anxiety; their hair, poor grooming suggests depression; how they are dressed, all dark colors suggest lack of confidence; their jewelry, loud or excessive jewelry suggests low self esteem - and so on.
Body Language - you cannot not communicate
People are signalling how they feel all the time. If you choose to pay attention you will learn that body language doesn't lie. NLP teaches that even when you are silent, you are communicating. How you stand, how you dress, how you arrange your feet, are saying something, whether you are aware of it or not.
I have had a fascination with body language most of my life. I get as much practice as I can, and not just with clients. Every time I am in a public place - on a bus, a hotel foyer, waiting in a shop - I spent the time studying someone. I am trying to learn what their actions, posture, clothes, expression, interactions with others is signalling to the world. Sometimes you get nothing much. But now and again their body language gives you a sudden insight into that person's life, what they feel, how they see the world. It is endlessly fascinating and when it is done right you get total rapport with the other person, you really understand how they feel.
After a while, you get a sense of general categories. I can now watch people go by, and think 'Depression', or 'Anxiety' or 'Unhappy marriage' just from the signals they are giving off. After a while it becomes automatic. It also gives you a much richer understanding of what is going on around you.
I recommend studying body language as a way to increase your rapport building skill with clients.