The ports should be grounded through the motherboard. The case should be grounded through contact with the PSU.

I've heard arguments it does some EMI shielding, which is probably technically correct, but with so many cases on the market having huge glass/clear plastic panels in their sides, leaving large gaping EMI holes, that doesn't seem to matter much.

That is what the screws are for.

Oddly enough i removed the one on my server because moving any cords plugged into the mobo caused the shield reset the computer.