Protects the environment from careless people. Have you just tripped over a tree root while wandering around in the woods? Or has an inviting forest path turned into a weedy field full of stinging nettle? Or is everybody around you carrying basketfuls of forest fruits while you have not even seen any traces of it? In that case you should search your conscience. It can drive a wood gremlin/sprite crazy when you, even unintentionally, leave a brown bag at the spot of your picnic.