What is the function of the boy in Waiting for Godot?

The boy stands for hope and anticipation. Didi and Gogo, as they wait (patiently) for Godot to arrive, hope for some “messenger” to assure them that Godot is still coming, and the boy’s arrival in Act I is some assurance that they are still in “Godot's” plans. Symbolically, he stands for our own belief that we have some purpose in life, that we are in fact part of a larger plan; the boy is like our moments of complete faith, our “religious” hope: most religions call for faith without proof. His arrival is a false epiphany, some sort of event that we, as humans, interpret as proof that our waiting for a purpose is not in vain. The very vagueness, the ambiguity, of the boy’s message is further evidence that Beckett is dramatizing these moments of (for him) false hope. His genius is having the boy (or a brother?) appear again at the end of the play (usually played by the same actor).  An important line is (paraphrased)
 "You did see us -- you won't come back here and say you didn't see us?"