The absence of Godot seems to serve the play on separate, yet converging levels. On one hand, Vladimir and Estragon should not see Godot. It makes sense for they discuss at length about him, explain their desire to see him, and their presence in the play is about Godot's presence. Not seeing him helps us understand much in the way of futility and human endeavor, as Vladimir and Estragon strive for something which is not to be realized. At the same time, this is what makes the play so very powerful. We only know of Godot's intimation, revealing little in way of actualization, but providing the backdrop for which all action can take place. In this respect, Godot is a totalizing influence whose presence is everywhere, and nowhere.
Godot's absence and ever-anticipated appearance advances Beckett's Existentialist philosophy. Essentially, for an Existentialist, life is absurd and meaningless; so in this short play, Vladimir and Estragon wait because they do not know what else to do, and it really does not matter if they do have plans, because there will be no meaning to those plans either.
This depends on how you read Samuel Beckett's existential play. Some interpret Godot to be a representation of God. If this is the case, it is important that Godot never appears in order to convey Vladimir and Estragon's blind faith in him. They continue to wait for him, even though they don't know where he is or what he wants. The boys who come in and out of the play are the "messengers" for Godot, but never bring any real news from him. At the end of the play, the audience sees Vladimir and Estragon vowing to leave; however, neither man makes a move to go.
It is important for Godot not to arrive. This important absence might be interpreted in thousands of ways depending on what one considers to be the real identity of Godot in Beckett's play. Godot is everything and nothing at the same time, a sacred all powerful absence as a force of life--a God, a dictator, the significance of life, a mother/father figure, order, beauty, love, the object cause of desire and so on.
The importance of Godot's non-arrival can be seen in a positive way as the dictatorial centre of power seems to be exposed in all its absence. It is an unacknowledged political lack in the master. This decentring may be seen as liberative as in Tagore's plays. The absence may also be positive if we read Godot as a meta-narrative that answers all questions. Its non-arrival enables Didi and Gogo to be more individualistic, more egalitarian and work with their apparently meaningless micro-narratives of action.
The failure of Godot in arriving may connote a dark vision of a hyper-real world where real signification has been complicated beyond existence, with the world turning complexly meaningless. It is the Derridean meaning--always deferred or the absurdist meaning--beyond the tentacles of logic.
Godot failing to arrive is as important as the fact that they still wait for him till eternity. This is the story of human courage, moving on in the void, working the way through the most restrictive of situations. That is Beckett's leason I think. One must go on, even as he knows he cannot go on.