They're to take the edge off the worst of the mud and stones when you're mtbing on muddy trails so it doesn't go in your face/up your back and back of your head.
They're not designed to do anything about dirty water.
Mine was more of a rhetorical question, but yeah they do that pretty damn well. If only I could reconcile my touring/commuting self (full mudguards with flaps) and my mountain bike/ATBing self (short plastic guards, anything else would probably snag and ground).
They're to take the edge off the worst of the mud and stones when you're mtbing on muddy trails so it doesn't go in your face/up your back and back of your head.
They're not designed to do anything about dirty water.