Years ago, in Mountain Bike Magazine, Uncle Knobby's column was asked about these things; Knobby replied that they were called "Big Wedgies", all bike shops stocked them, all you had to do was go there and ask for one...lol!

They are there to keep the chain from destroying the spokes, should the chain fall into them. I don't see, and never HAVE seen, one of those saving a derailleur.