Side skirts - do they actually hang down and help create low pressure under the car?

[steved0x]

Or are they primarily for looks, to dress up the sides of the car, presumably to matchup with a front bumper like the GT3 bumper?

For fun I was looking around on eBay and found these fiberglass skirts - the price seems right, but wondering if they actually do anything beside looks?

PORSCHE 986 BOXSTER SIDE SKIRTS GT3 SPOILER | eBay

Has anyone ever fit these style of aftermarket side skirts? I got a front bumper lip as a gift and so if I put it on, I might want to add some sort of side skirt treatment to keep it more balanced.

Thanks

Steve