Why is land so important to the people in the movie The Grapes of Wrath?

Your question specifies the movie adaptation of The Grapes of Wrath, but the Steinbeck novel on which it is based addresses the same relationship between people and their land.

Land is important to the Oklahoma sharecroppers because it has been their source of life and meaning.

As farmers, the men in Steinbeck's seminal novel have worked on their land for long hours each day during planting and harvesting seasons. The earth they have tilled has produced life, fed them and their families, covered their hands and faces, and gone into their skin. To these men, the earth is a living thing, not just some surface of the world. It is an integral part of their existence.

When the corporations decide the sharecroppers must go, one of the farmers, Muley Graves, refuses, saying, "Fella gets use' to a place, it's hard to go." In Chapter 9, Steinbeck writes the thoughts of the farmers as they think of the rich land in California: "We'll start over."

But you can't start. Only a baby can start. You and me — why, we're all that's been. The anger of a moment, the thousand pictures, that's us. This land, this red land, is us; and the flood years and the dust years and the drought years are us. We can't start again.

These men define themselves by the land. Because their identity belongs to the earth, Grampa does not travel far before he has a stroke and dies. After the men bury him, Jim Casy remarks, "Grampa and the ol' place, they was jus' the same thing." Even though the old man spoke of how he would squeeze the California grapes over his head and let them run down into his whiskers, Casy says,

He was foolin', all the time. I think he knowed it. An' Grampa didn't die tonight. He died the minute you took 'im off the place... He was that place, an' he knowed it...

He's jus' stayin' with the lan'. He couldn't leave it.

Indeed, there was a spiritual bond between the men and their land. Along with this bond, the idea that they have no job and no way to care for their families deeply disturbs the men. The land is what has given these men hope and pride when they have good harvests. Without the land, these men have nothing certain.