There's a plastic disc between the cassette and rear wheel on my bicycle. It doesn't seem to have any effect on the actual drivetrain; but I've seen it on a number of bicycles.
What is it for?
From Sheldon Brown:
A plastic or sheet-metal disc that fits between the cluster and the right-side spokes of a rear wheel. This is intended to prevent the derailer or chain from getting caught in the spokes, possibly causing very extensive/expensive damage/destruction to the wheel, the derailer, and the frame.
A spoke protector is not a necessity on a bike that is well treated, because the derailer can't go into the spokes if it's properly adjusted and if it is not bent. Bicycles which are subjected to rough handling, however, are prone to getting the rear derailer bashed in, and in such a case, the spoke protector can prevent very serious damage.
And from purely personal anecdotal experience: I have a friend that doesn't like spoke protectors and has been pretty vocal about it. Faith would have it that a maladjusted XTR derailleur wrecked his wheel and destroyed itself. A spoke protector would have prevented just that.
The other reason for the guard is to help keep chain oil off the brake disc. Once the pads are fouled with oil the brake is useless. Sloppy oiling of the chain can cause a spray of oil droplets to migrate to the disc more easily then you might think. The guard does help prevent this. It’s worth keeping it on the bike if you have rear disc brakes. And oil conservatively, slowly turning the crank for a bit then wipe off excess oil before riding.
It's a dork disk.
It's called that because a lot of cyclists are dorks and therefore remove the disk. Eventually, due to rear derailleur damage, the rear derailleur gets out of adjustment and causes the chain to get between the spinning wheel spokes and the stationary cassette. You get spoke damage that way.
In contrast, non-dork cyclists have a completely different approach: they not only leave the dork disk there, but also buy these disks for self-built wheels. One disk doesn't cost much but can prevent lot of damage due to chain getting between the spokes and the cassette.