First, The Grapes of Wrath is an American classic. It allows the reader to enter the world of the depression, dust bowl and the 1930's in a way that you experience the desperation of people during that time period.
It is a great way to combine American history with literature. My 6th graders viewed the movie after we studied the Dust Bowl and even though it is in black and white they were moved by the story. Some even asked if and where they might rent it to view again.