Internal plastic cover over amp heatsink, why is it there?

I'm running a semi-vintage Pioneer digital amp, last weekend I removed the cover to clean a penetrometer that contained dirt, plus general cleaning the amp out.

In the center of the amp is a very large heat sink, i'm guessing about 8 x 4 inches, then maybe 4 inch in height. Connected directly on top of this heat sink was a large plastic cover, I removed the cover to clean but then re-fitted. This cover overlaps the heatsink by about 2 inch each side.

Does anyone know why the cover is over the heat sink, am I correct in thinking it's to spread heat and prevent a heat built up directly above the amp in one place. If so I presume this would be an issue if units were directly stacked on each other, however I have a glass hi-fi stand and the amp sits in it's own space. Because there is no risk of heat affecting anything else am I best just removing the cover.

What is the heatsink connected to? some regulators, etc. have their tab/heatspreading surface tied to ground or the negative supply, etc. and it may be there to prevent short circuiting either in some variants of the design and/or if the chassis is grounded and the heatsink tied to one of the supply rails.

Hi Rroff, the heat sink has a large (least 1.5 inch) single transistor connected to the side. Its one of those oval flat types with its own small heat sink that connects to the larger heat sink. There maybe other things at the base, but that large transistor is the main thing.

I did wonder if the cover was protecting the heatsink from touching the top case, but the cover is screwed on with metal screws that are exposed upwards towards the cover. If the cover was ever pressed in, it would hit those screws with the cover still in place.

None of this is really important but was just curious why Pioneer would cover the heatsink up, it reminds me in some ways like the under bonnet sound installation you have in cars.