Origins 

Little is known about how the Bag Men came into being, except that they have some sort of spiritual/magical link. The Bookburner reveals that the idea for the Bag Men was created by his brother Mr. Revise and that he did not believe that they would be successful due to one large fatal flaw: if their bags are cut, they die.  Regardless, Mr. Revise creates them to be guards at the Golden Boughs Retirement Community to help keep the inmates from escaping. However, this idea is unsuccessful when Jack Horner comes to the facility and plans a mass breakout. 
 
Later, when the Bookburner comes to the Mundane world with his army to attack the facility, Mr. Revise and the other Librarians unleash the Bag Men. They are helping to stave off the attack until the Bookburner remembers their weakness and sends in his Knifejohns to cut them to shreds. 
 

Abilities

 
The Bag Men are an incredibly useful asset due to many factors. They need no food, rest, or sustenance; when they are not of use, they are

 zipped back up in a duffel bag; they are incredibly strong;  and they are loyal to a fault. The only weakness that they have is that the bag that contains their essence is easily torn.