Here's why they're used:
* Blood Thinning: Leech saliva contains hirudin, a potent anticoagulant that prevents blood clotting. This makes them useful for treating blood clots and improving circulation.
* Reducing Swelling: Leech bites can also help draw out excess blood and fluid from an area, reducing inflammation and swelling.
* Reattaching Tissue: Leeches are sometimes used to help reattach severed limbs or fingers by preventing blood clotting and promoting circulation in the injured area.
While leeches are parasitic, they're carefully controlled and used in a medical setting. They're not simply "feeding" in the way a mosquito might, but rather acting as a medical tool.