Akwat Kbrana said:
IIRC, the reason for the slugs is that yellow lights were mounted on the sides of Palpy's hood that shone onto his eyes, thus making them "glow." Unfortunately, the lights themselves (clipped onto the hood fabric) were visible in the footage, so ILM rather crudely painted them out, thus producing the black globules often referred as "the Emperor's slugs."

Nonsense!

Here's the real reason, the one too terrible to consider.

The slugs ARE the Darkside. I bet Vader had some under his cape.

Think about it. It all makes perfect sense.