The significance of this scene is in the fact that the witches foretold that Macbeth would die only when the Birnam wood comes to him. They didn't specify the way that would happen, and when this happened, Macbeth knew that his time has come.
The one purpose of the personification in the above excerpt is: it makes death seem friendly and familiar. The poem "Because I Could Not Stop for Death" is about the journey of the poet towards death. She tells about the appearance of the death and her journey towards her grave.
