If you are referring to Harper's interaction with Prior in the dream sequence of Act I, I think that her primary motivation is to better understand her state of being in the world. The idea of "the threshold of revelation" is something that is profoundly important in both this scene and to her character, in general. Harper struggles with having to break away from the conformist vision of the Mormon world, the expectations placed upon her as being a woman, and the idea that she recognizes that there is something amiss in her marriage. All of these are forces that exist inside Harper, yet she lacks the vocabulary and the pattern of recognition to articulate this condition in the world. For Harper, seeking to identify what defines her state of the being in the world is extremely important. This is why the scene with Prior is filled with this desire to gain insight into oneself and one's world becomes a part of her character. When Prior "outs" her husband, it is a critical element in this process. Finding out that Joe is gay helps to bring much in way of understanding, something that drives Harper's character throughout the drama.