Thornton Wilder reveals his sense of humor in this scene at the end of Act One: "Daily Life" in which Rebecca Gibbs tells of a letter her friend addressed, ultimately, to God.
Grover's Corners seems the center of the universe for the Gibbses and the Webbs. Little do they know that in the grand scheme of things, throughout the history of time, they are but grains of sand on a beach.
This letter shows the hierarchy of the universe from a child's imagination. Whereas most adults confine themselves to daily life (chores, meals, work, sleep), children have a better grasp of the eternal because they have not been conditioned by routine. The address not only contains Grover's Corners, but it extends beyond the temporal into the heavenly, revealing a spiritual dimension.
In Act III: "Death," the play crosses into this spiritual dimension during Emily's funeral and her birthday revisited. Having spent one last day among the earthly, she sees how programmed her family is by routine and how they fail to grasp the big reality of the meaning of life, the cycle of life, and the hereafter. Depressed by this, Emily longs to return to the dead and "live" forever in the spiritual realm.
So, this letter in Act I comments on and foreshadows the eternal themes Wilder develops further in Act III.