This secures the base of the ladder to the object on which the ladder is resting. For example, if this object is more structural like a building, then you’d anchor the ladder strap into the side of the brickwork. Or on the other hand, if you had a pole, you would simply loop the strap around the pole and re-attached it to the yoke. This is a way of giving an additional method of safety when working on ladders at height.