Well, for leeches you don't eat them
Leeches have a substance in their saliva that keeps the blood from coagulating - i.e. when the leech bites a creature, it makes sure the blood keeps flowing and doesn't stop up. So they use that substance in cases where they need the blood to keep flowing smoothly.
For maggots, they are traditionally critters that eat dead flesh. So in a situation where a person has damaged tissue, they sometimes put maggots on that area to eat away at the damaged tissue so the rest of the area can grow properly.
I don't know that I think either is gross really, is a lion gross? He eats dead animals. All creatures eat something
It's only us humans that say some things are OK to eat and others are not OK ...