To me chapter 15 was the most important to me because it shows you how hard times were back then and it shows how generous and nice people were back then. I believe the theme here is just helping people. Mae and Al helped the men when he came in to get a piece of bread and gave the kids candy, two for a dollar, and at the end the trucker who saw everything left Mae a pretty good tip.
"She held the screen door open and the man came in, bringing a smell of sweat with him. The boys edge in behind him and they went immediately to the candy case and stared in—not with craving or with I've or even with desire, but just with a kind of wonder that such things could be. They were alike in size and their faces were alike. One scratched his dusty ankle with the toe nails of his other foot. The two whispered some soft message and then they straightened their arms so that their clenched fists in the overall pockets showed through the thin blue cloth."
I really liked this passage because it is really descriptive you can really see the scene going on in your head as you read it. It gives you an insight inside this family's life.