Maggots on animals is called FlyStrike or Myasis, The maggots that hatch eat both dead and living tissue, this causes further infection and infestation. Maggots on animal is not good for infection - they need to go asap or they can kill the animal.
Maggot therapy used in humans is a controlled therapeutic use of maggots that have been hatched in sterile conditions, they are "medical grade" maggots and placed in the wound with special dressings which keep them on the tissue they are to feed on. I believe they are only designed to feed on necrotic, infected tissue where as maggots laid in a backyard/barnyard setting will feed on anything.
Hope that helps clear it up.
Thank you for the explanation. Very good feedback.