How is the relationships and fostering of friendships with Lou Ann important to Taylor? How does she change and grow through this relationship ?

The relationship between Taylor and Lou Ann is very important to the plot of The Bean Trees (1988), Barbara Kingsolver's first novel. Let's go over a few of the main ways that this friendship is crucial for Taylor, the protagonist, before discussing the ways that she changes between the moment she meets Lou Ann and the point where the narrative ends. 
First: Taylor is alone and faced with a major challenge at the beginning of the book, and Lou Ann offers her support and companionship. At the start of the story, Taylor leaves her mother and her home in Kentucky to head off on an adventure. She's immediately faced with a challenge when she stops to eat at a roadside restaurant in Oklahoma and a stranger pushes a baby into her arms. Taylor doesn't know what to do. She decides to care for the baby, but she's alone and doesn't know anyone in town. Until, that is, she meets Lou Ann, a local young woman who posted an ad in the local paper in search of a housemate to share expenses. Lou Ann becomes Taylor's friend and a point of contact in a community where Taylor is an outsider.
Second: Lou Ann is a mother, so she's able to teach Taylor how to care for a child (Turtle, in this case, the baby that was handed to her in the restaurant.) And since she's already staying at home with her baby, she's able to offer live-in childcare for Turtle, the baby that Taylor has informally adopted, while Taylor goes to work. In both practical and emotional terms, Lou Ann becomes like family to Taylor.
Third: Taylor's relationship with Lou Ann helps Taylor to grow up. Personality-wise, Lou Ann is different than Taylor, and their complementary characteristics help both women to learn and grow. Lou Ann is more maternal and traditional than Taylor, who's initially more free-spirited and interested in adventure. In important ways, Lou Ann models what it means to be a responsible adult.
This third point leads into the second part of your question. How does Taylor change in the course of her friendship with Lou Ann?
She becomes a mother, in a sense, even though she has never given birth to a child: watching a nurturing woman like Lou Ann care for her own baby, she becomes a stronger mother to Turtle. Conversely, Taylor builds confidence as she helps Lou Ann to become more independent and go on her first job interview.
In her friendship with Lou Ann, Taylor realizes the importance of female friendship, support, and community. She learns that "family" can come in many forms—that family can be something we choose, not just what we're born into. This idea is well-expressed in the following passage from the novel, in which Lou Anne is speaking to Taylor:

Taylor, remember that time you were mad at me because you didn't want us to act like a family? That all we needed was a little dog named Spot? Well, don't get mad, but I told somebody that you and Turtle and Dwayne Ray were my family. Somebody at work said, 'Do you have family at home?' And I said, 'Sure,' without even thinking. I meant you all. Mainly I guess because we've been through hell and high water together. We know each other's good and bad sides, stuff nobody else knows.