People go for retreats for various reasons. For those attending a Buddhist retreat, the purpose is not about learning more about Buddhist psychology or a particular sutra. Its purpose is to help us untie the knots that we possess within us.

These knots are mainly of 2 types: i) our concepts and ideas and ii) our afflictive emotions. All of us have concepts and ideas. When we become attached to them, we loose our freedom and open mind. When this occurs, we are no longer in touch with the truth in life. The second type of knots that we possess is our afflictive emotions such as fear, anger, arrogance, despair and discrimination.
If we wish to be in touch with reality, we need to remove our concepts and ideas. If we wish to be happy and be free, we need to overcome our destructive afflictive emotions. One quick way of doing so is to go for a retreat.
During the retreat, when we listen to a Dharma talk, we are not polluting ourselves with more concepts and ideas. Instead, the actual purpose of listening to a Dharma talk is to help us release the concepts and ideas that we hold on strongly. We listen so as to get free from all notions and concepts.
Source: "Answers from the Heart" by Thich Nhat Nanh