Prevention is always better than fixing problems after the fact; flossing is better than getting root canals.
Premarital counseling is meant to help couples address issues before they become problems. There is a practical reason for premarital counseling–to reveal and examine topics that might not have been discussed before this point. Dating couples and even couples that have already been living together may know each other very well and have even been through all kinds of challenges and hardships together, but there are certain topics that don’t typically come up until after marriage, especially conversations about combining finances, the involvement of in-laws, and expectations for retirement. Then there is the information that should be revealed and discussed before marriage because the other person should know what they are getting into–physical and/or sexual abuse from past, criminal history, sexually deviant tendencies, family history of addiction and/or alcoholism, and family history of mental illnesses and hereditary illnesses.
These are difficult conversations that are understandably avoided, but they must be discussed to ensure both people are prepared for spending their lives together.