Because they’re significant? Freudian psychologists are interested in a patient’s dreams. Not all psychologists are. Dreams are usually unconscious constructs of thoughts that you had while awake (even fleeting ones that you don’t realize) while you sleep. Some people like to believe, when they dream of a dead person, that the person is deliberately coming to them, to tell them something important. Frequently, people who believe that, have issues with death and dying. Sometimes it’s cultural (as in the dream scene in “Fiddler on the Roof”.