Two fold, first it made the are fuel proof, second if a bit of the engine compartment was seen thru an air inlet/outlet it was less noticeable because it was black and would be seen as a a void, a shadow, part of an engine, or whatever else your brain tried to fill in the blank with. Its like when you install paneling next to a natural stone fireplace. It is impossible to cut the paneling perfectly to such an irregular surface. So you first paint the edge of the wall around he stone so when you cut the paneling any slight gaps show up as shadow. Black paint can hide a multitude of sins when used correctly. My wife's Uncle Eddie (a retired A&P with AA) taught us that.