What is the purpose of the Grand Canyon story?

Father Walls promises Jeannette that he will stop drinking. He ties himself to his bed for a week to withdraw from drinking. To celebrate his sobriety, they take a trip to the Grand Canyon. Their old car doesn't make it and the family is forced to walk back to Phoenix. A woman gives them a ride and offers the kids sandwiches. Dad feels humiliated and begins to drink again. This event represents the fragile mental health Mr. Walls has. The precarious line between stability and instability is always a factor in Jeannette's life.