I don't have an answer myself as to "
why" but it would stand to reason that the insulation would absorb heat (to protect the hood/paint) as well as provide
some barrier to engine noise to the outside...but in regards to it's use as a noise barrier it would seem more useful on an "open" hood (shaker hood) than a closed hood (base flat hood), yet they were installed on the latter
Again, they were used on flat hoods and turbo hoods from the factory but not shaker hoods.
As to
why not shaker hoods...I for one am not going to lose any sleep trying to figure out the decisions that automotive engineers (past or present) make