What makes the use of metaphor in Beckett's works and specifically this one so unique is the way that he uses metaphor to connect his work to his very pessimistic view on life that presents his harsh and unyielding beliefs about humanity and what life is all about. 

The whole play could be viewed as one extended metaphor about life and the way that we live it full of depression and misery. Life, more in this work than in Beckett's other plays, seems to be nothing more than one endless journey of sadness and pain. The use of metaphor by Beckett is therefore something that allows Beckett to profoundly challenge his audience with their assumptions about life and to express his Absurdist viewpoint through comparing life to being about nothing more than ashcans and rubbish or being dictated to by somebody else and being dependent on others even when you hate them.