Forces must be specified in world coordinates. For a lander, the thrust forces are most easily specified with respect to the body (lander) frame. Therefore, they must be converted to the world frame by multiplication by a transform, which occurs in the body2world function.
Note also that in this application, torques are explicitly applied to the lander. It would be worth trying the applyForce(force,relpos) function to avoid this requirement. Note that while relpos is the position relative to the origin of the object, it is specified in world coordinates, and so is force.