What are these plastic grounding washers for?

[Cawbuk]

Instead of grounding straight to the chassis, all the engine bay grounding points have these plastic washers.

I asked a Vauxhall mechanic if I could just remove the one from the battery ground cable so that I could add an extra 1/0 awg for car audio. "Oh no", he said, "Those are important because they... oh, I can't think of the word!"

Can anyone think of the word?! Are they to prevent corrosion from scuffing the paintwork for a traditional connection?