The value of having alternative voices is that Steinbeck can change the tone and mood of the point of view to suit his purpose, whether it's to include social and historical information or to dramatize what the Joads are feeling.
Through the different voices, he is able to better reflect the theme of the novel. The Joads represent the need of all men and women to find some kind of dignity in their lives. These chapters allow him to give the reader necessary background information of the Depression era so we can understand its effect not only on the Joads but also on all of the migrants like the Joads. When Steinbeck wants to focus on the Joads in particular, the point of view then becomes more restricted, allowing the reader to feel what the Joads feel.
This method also allows Steinbeck to include much of the symbolism and conflict in the novel. In Chapter three, the journey of the turtle crossing the highway is symbolic of the journey of the Joads and other migrants to California, demonstrating their will to survive. We also see the conflict between the migrants and the bank.
By changing the narrative between different points of view, Steinbeck is able to tell his story more dramatically. It provides a way to include the literary elements of any work of fiction so it's more enjoyable to the reader.