Why does skull twins from space police III have a two sided head if he wares a helmet anyways?

[Foreman]

Why is this and is there any other minifigures that have this

I think Slizer might have it too but I can't remember. It was the Chima figures that have double faces, not Slizer, well he might, I don't have him

Edited July 17, 2018 by Foreman

[Johnny1360]

Quite a few are like that, they usually come with an optional hairpiece as well. 

[Peppermint_M]

Also, the eyes can be seen inside a helmet and have different expressions, so it adds that extra dimension of play to a figure. There can be storytelling reasons, or for the more generic characters, the one printed side is one character, while the other print is another. Like with the mummy figures in Pharaoh's Quest:

These heads then supplied all the Mummy mooks and allowed some variety.

[Artanis I]

Lord of the Rings orcs with 2 faces and a helmet, although I don't think their whole face was covered, and also not all of them would wear a helmet, some had hair over their backface instead

[The LEGO Communist]

In case you want to have them without the helmet. 

[MAB]

And flexibility so that they could be used for other characters, whether they later decide to or not.