Auto Print Unprinted documents at the picking screen can be a useful feature if clients have uploaded printable documents or labels to orders after they have been printed or picked. Also useful if you have product specific order documents added to order that you do not want to print until the packing stage (e.g. instruction leaflets, warning labels, returns documents etc).
This setting is enabled on a per user basis to enable this setting:
User Settings -> Picking Defaults -> Automatically Print Unprinted Order Documents on Scan Actions Page