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.