Not everything that grows since the fall and expulsion from Eden, has a purpose that’s healthy for humanity. Crabgrass comes to mind. Sin brought things into being that Yhwh God never intended to exist in this world, that do harm, and NO good at all. Mosquitoes are another example of such. They serve no useful purpose. Flies’ maggots at least help deal with decaying matter, but the flies themselves not so much. Just because it’s here and natural, doesn’t necessarily make it good for us. Satan is called “Lord of the Flies” for a reason.