That Thing was a CPSC mandate from back in the 70's (We know that CPSC was/is the Fount of Cycle Maintenance Wisdom), we called them Pie Plates when they were sturdy chromed metal. Yours is an inferior ( if that makes any sense in this context) plastic variant that has failed in service, it must go.

They are the Answer to the Question That No One Asked. If your rear derailleur is adjusted properly it serves no function.

Go to your LBS and have it exorcised and your derailleur adjusted properly by a person, or better yet get the tools as referenced previously and learn to do it yourself in person.

Being able to properly maintain your equipment personally is "a good thing", not some ill conceived chunk of rotating plastic.
Several other Good Things like having gears that shift accurately, brakes that stop and tires that are not flat may follow.

Personally,

-Bandera