What is the significance of the conversation between Lady Macduff and her son?

This short scene from Act 4, sc. 2, shows how Lady Macduff feels about her husband's absence and shows the general current mood of Scotland.  Lady Macduff is angry that her husband has left them in Scotland to go to England.  She implies that things are so bad in Scotland that she feels vulnerable and unsafe with him gone (she obviously has good intuition).  She tells her son that her husband is dead, which, in a way, he is to her right now.  Her son knows better and realizes that his mother is simply angry with her husband and is speaking metaphorically.  Ross, her cousin, tries to soothe her, but she says if her husband loved her and his family, he would not leave them alone.  Later, in the next scene, when Malcolm tests Macduff's loyalty to Scotland, that loyalty is clearer to the audience who has seen how Macduff left his family in harm's way to help his country.