I agree with the Bike Snob that pie plates are about as misplaced on many bikes as the red skinsuit man, but they do have their utility.
Pie plates are plastic rings placed against the spokes on the rear wheel of dérailleur equipped bicycles. They protect the wheel and derailleur against damage by preventing the chain or dérailleur from going into the spokes if the rear dérailleur is misadjusted. I once had to replace spokes, chain, derailleur and chainring for want of a spoke protector because my dérailleur hanger got bent in on the train.
If you ride a high end road bike, you’re expected to fiddle with the limit screws like a tweaking meth user, so get rid of the pie plate. Ditto for fixed gears, singlespeeds and hub geared bikes, which have no need for spoke protectors. If you ride your bike for transportation, though, and your bike is flung against other bikes, crammed into closets, dropped on the sidewalk and you otherwise don’t want to mess with the gear adjustments, keep that spoke protector in place.