How does Freud justify the necessity of defense mechanisms to mental health?

Defense mechanisms are unconscious denials or distortions of reality.We are lying to ourselves when we use these defenses,but we are not aware of.If the defenses are working well,they keep threatening or disturbing material out of our conscious awareness.As a result,we may not know the truth about ourselves.We may have a distorted picture of our needs,fears,and desires.
There are situations in which the truth about ourselves emerges,when our defenses break down and fail to protect us.This occurs in times of unusual stress.If we knew we were lying to ourselves,the defenses would not be so effective.When the defenses fail,we are stricken with overwhelming anxiety.We feel dismal,worthless,and depressed.Unless the defenses are restored,or new ones formed to take their place,we are likely to develop neurotic or psychotic symptoms.Thus,according to Freud,defenses are necessary to our mental health.We could not survive long without them.