To limit the probability that the materials used to construct a firewall will be easily altered or damaged during use, which could lead to an inability of the firewall to control the spread of fire from one building to another, which could lead to harm to persons in the building not originally involved in the fire.
To limit the probability that the materials used to construct a firewall will be easily damaged by falling debris during a fire, which could lead to a failure to resist the spread of fire from one building to another, which could lead to harm to persons in the building not originally involved in the fire.