Plastic "pie dish"

[OakLeaf]

It's there in case the chain would come off and get stuck between the cassette and the spokes. If that would happen, the wheel would seize.

It's a very rare occurrence, since the low limit screw on the RD as well as the whole design of a derailleur system should prevent it. Still, if it does happen it could be catastrophic, which is why manufacturers are required to put the pie plate on. But they're at best messy, and at worst clattery and noisy, which is why most people take them off.

There's also a bit of peer pressure, as they're considered geeky, but obviously that's no reason to do anything; and weight weenies like to save the couple of dozen grams.

I'd ignore it unless it's making noise or making it unreasonably hard to clean your cassette.