To limit the probability that the absence of grab bars will lead to a person with a physical disability slipping or falling, which could lead to harm to the person.
To limit the probability that a person's arm will slip between the grab bar and the supporting surface, which could lead to the arm becoming trapped, which could lead to harm to the person.
To limit the probability that a grab bar will have insufficient resistance to loads, which could lead to the collapse of the grab bar, which could lead to persons falling, which could lead to harm to persons.