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 the subject building to an adjacent building, which could lead to damage to an adjacent building.
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 the subject building, which could lead to damage to an adjacent building.