Final answer:
Steinbeck includes Bill Tenner's letter to portray the impact of the Dust Bowl and the challenges faced by the characters in The Grapes of Wrath.
Step-by-step explanation:
Steinbeck includes the part about Bill Tenner's letter in The Grapes of Wrath to highlight the impact of the Dust Bowl on the Joad family and other families like them. Bill Tenner's letter describes his difficult circumstances and how he lost hope in finding any work. This part of the story emphasizes the harsh reality of the Great Depression and the challenges faced by the characters in the novel.