I brought this up at a party a few years ago. My friends and I still can't come to a good consensus. Did they offer any protection at all? Did they lock? Why even have a door there at all? Were they as prevalent as they seem to be in films?
here's a previous thread:
Top answer basically says that the were present in warm climates, but they always had full sized doors to shut when the shop was closed (you can actually see tall doors backing up the small ones in this clip from back to the future).
Thanks to /u/atomfullerene for supplying the link to a previous discussion. I was one of the players in that discussion. If you have additional questions, let me know.