For the first twelve years of her life, Esperanza lives the life of luxury. We kind of imagine it like this. But when she and her mother lose everything and have to flee to the United States, Esperanza gets a crash course in poverty. She learns about the connection between class and race, the relationships between the rich and the poor, and the quality of life for poor workers. What's more, she learns this all first hand. Talk about the biggest reality check ever.
If Esperanza had continued to live her life of luxury and privilege, she never would have learned that many people live in poverty and struggle to survive. Losing everything makes Esperanza a more knowledgeable and sympathetic character.
The novel makes the argument that the unfairness of working conditions on the big farms is one of the causes of poverty.