Why do people wear retainers before braces?

If someone is wearing a true retainer before braces it is normally to maintain space following primary tooth extraction. However, the more common use of a "retainer" before braces is for arch development. When a patient has crooked teeth it is almost always because there is not sufficient space in the mouth to accommodate straight teeth. There are only three ways to remedy a shortage of space: 1. extract teeth, 2. slenderize teeth, or 3. enlarge the dental arch. A "retainer" before braces is typically NOT a retainer but rather an arch development or arch enlarging appliance. This appliance is worn to eliminate the need to either slenderize or extract teeth. I hope this answers your question.