I/O shields act as ground path for rfi or magnetic induced electricity/static electricity picked up by shielded cables. So while not exactly a necessity, they are functional for more than just sealing up the cavity.
Dells are very much proprietary. Probably the worst when it comes to ownership and prevention of tampering with aftermarket parts. That includes everything from motherboards to bios to periphials to even Windows itself.
Basically, to replace a 3020 motherboard, you need another 3020 motherboard, which are all the same per model. A 3020 slim can different to a 3020 mid etc, but the same motherboard in a 3020 mid might be used for several different mid models.
None of which line up with aftermarket parts. Just like ypu can't use a Gigabyte I/O with an Asus mobo.
Or a Dell. Which has windows tied to the serial tag, ultimately the motherboard. If you use an aftermarket motherboard, you'll need to buy a new Windows key because you'll no longer have a Dell pc, you have an aftermarket pc in a Dell case, and Dell isn't about to let you get a freebie key at their expense.