Not really - provided gears are adjusted well. Proper name is a spoke protector
or pie dishColloquially known as a 'dork disk'
or pie dish
easy and cheap to replace and widely available on the net advertised as a spoke protector or similar and it is to simply stop your chain over shifting when changing into bottom gear and getting tangles up in the spokes and potentially buggering the rear wheel. but as said if gears are set up properly them no need for it
Cheers Ed