To limit the probability that the incinerator spark arresters will become obstructed, which could lead to improper operation of the incinerator, which could lead to a fire or explosion, which could lead to harm to persons.
To limit the probability that damage to the incinerator will go unnoticed [e.g. burnt-out arrestor with large holes], which could lead to the unwanted release of sparks, which could lead to ignition of adjacent combustible materials, which could lead to a fire outside the incinerator, which could lead to harm to persons.