Babies need to know that there is someone there to love them, care for them, and provide for them when they are in need. So when babies, especially young babies, are crying in the middle of the night, they are seeking that love and support. They shouldn't just be ignored.
When a baby cries, that means one of its basic needs is not being met. A crying baby is either hungry, soiled, tired or in pain and it needs immediate attention. Letting a baby "cry it out" as suggest by Dr. Ferber can be detrimental if the baby needs immediate attention before a medical malady sets in.