It serves the purpose of allowing me to leave my vest off for a few minutes while making a quick trip. I always carry OWB on my right hip due to too many years of experience reaching there for my firearm. For work and school, where I am not allowed to OC, I wear a vest to cover my weapon. OC allows me to not grab my vest when I run to the store or go to lunch.
It also allows everyone who carries concealed to not worry about if the wind blows their jacket open or if they are printing. I know the law allowed that by requiring an intentional display to be illegal, but we all know of people who were stopped or hassled because of an unintentional display. This problem was solved by allowing OC.