So instead of dropping blocks, they will place them, or if there is a block in front of them, take them. Great for automatic farming, another option for making doors, regenerating walls assuming block breaking is turned off somehow, use as sensors, use with cobblestone generators. Maybe make it where if there is a diamond pick inside its inventory it can break obsidian. ect... Perhaps using a sign with a command so that they can either place/break blocks or both.
