Why does Steinbeck (the Author) describe the setting with such detail?
The author of "Of Mice and Men," describes the setting with such descriptive detail. As a way to interact with the reader to feel part of the setting. For example, on page 2, chapter 1, "Evening of a hot day started the little wind to moving among the leaves. The shade climbed up the hills toward the top. On the sand banks the rabbits sat as quietly as little gray, sculptured stones." This demonstrates the setting as setting the mood for the reader of feeling the moment.
Not all also that interacts with the dialogue. As said in page 3, "His huge companion dropped his blankets and flung himself down and drank from the surface of the green pool... 'Lennie!' he said sharply." This demonstrates how the reader can understand why the character respond in that certain way.