The vestibule of hell is reserved for the uncommitteds or neutrals.
The uncommitteds or neuatral are those who chose no side. They chose neither god nor the devil; consequently, they do not belong to heaven nor hell. What is more, they get to follow aimlessly a flag with no emblem, symbolizing that they followed nothing while living, so they follow nothing worthy in the afterlife.
The vestibule of hell dramatizes Dante commitment to commitment.