The posts above nicely sum up the variety of views that this play offers. We can say that the characters are waiting because, in a way, that is all there is to do. They are waiting for something that is not coming, which I take to be a pronouncement of meaning. They are waiting to find out why they are waiting. But nothing is going to come. This is the logic of the play, a challenging logic in the context of narrative drama.
The waiting is significant because it represents the substance of the lives of the characters. The "person" they are waiting for is important only insofar as he/it is imbued with a power that the character on stage do not have, i.e., the ability to categorically and finally pronounce or provide a meaning for...well, it's been said, life.
The waiting is symbolic, or metaphorical. They are just whittling away their lives. However while they wait, they are interacting with each other and with a few other brief characters. The meaning of life is that there is none. Life is just life!
This play has been the subject of much analysis by literary scholars and students. Some have speculated that the title, 'Waiting for Godot," is a play on "waiting for God," and that the characters are engaged in an existential crisis in which they are searching for the meaning of life, including whether there is a God or any higher power in the universe. Other theories about the play's meaning include the possibility that it may be a metaphor for life itself, and that the ultimate event at the end of waiting is death. The landscape, composed of one bare tree, is symbolic of the Tree of Life which encompasses all aspects of human experience and the potential for experience and transformation.