Back in the day there was no backplane and the only port on the mobo was the AT keyboard port so that was the only hole in the case. The rest were punchouts for parallel and various serial ports that would be connected to the mobo via ribbon cable. When the first ATX mobos came out they kept the punchouts for the backplane but that required all the manufacturers to use the same port layout so that lasted all of like 2 years before the pop-in shield became the norm.

How are the new ones getting around the different port layouts?

stops airflow

Keeps the I's O.

Real answer: it serves two purposes. First it ties the ground shielding from the ports to the grounding plane of the case itself so that static discharge is dissipated there rather than the motherboard. Second it completes the RF shield created by the case, this was way more important in earlier in computing and is also required to comply with that FCC rule about not interfering with other devices that you see printed on the bottom of things still sometimes.

It is the underwear for the PC. The PC's nethers will be left uncovered. Although that might be intentional in this case since some people like the breeze in their nethers.