If/when your chain goes over the top of the big sprocket, the plastic spoke protector keeps the chain from cutting and gouging the spokes. That may prevent some people from not knowing and still riding until the spokes break (maybe a crash). It may prevent some people from respoking a wheel, it is a pain.
Most people will now tell you it has no use if your derailleur is adjust properly, so take it off. They are wrong, lots of things can cause the chain to go over the top. Most people take them off. They are even called "dork discs" because they seem so silly. Do what you want, the risk is a bunch of spokes, or a wheel rebuild. Possibly broken spokes in the middle of nowhere, but unlikely.