To answer this question, take a look at Helen Keller's autobiography, The Story of My Life. In Chapter Seven, Helen talks about her teacher, Anne Sullivan, and says:
My teacher is so near to me that I scarcely think of myself apart from her. How much of my delight in all beautiful things is innate, and how much is due to her influence.
To see this influence in action, take a look at Chapter Six, in which Helen recalls some of her experiences with Anne Sullivan. She says, for example, that Anne taught her how to understand "abstract concepts," like 'love' and 'think.' This had a profound impact on Helen since it made her feel connected to other people in the world, as though there was an "invisible line" stretched between their souls and her own.
In addition, in the next chapter, Helen talks about how Anne Sullivan developed her love of the outdoors, through studying outdoors and enjoying walks together. It was also around this time that Anne taught Helen to read, an act which fostered Helen's love of learning.
Anne Sullivan, therefore, played a pivotal role in Helen's personal and educational development, which is why her influence was so important.