Leeches do have benefits for helping dying tissue, although not as many as medieval doctors may have hoped.
Nothing says old-timey medicine like leeching, or applying leeches to the body on purpose so that they suck up all the blood and balloon up like the slimy, disgusting creatures that they are. Leeching in medieval times was used to treat many different ailments and shared a certain similarity to bloodletting, but was even more popular.
Fast forward to modern times, and doctors still use leeches, which have anti-coagulating effects and can indeed help clean the blood. They have proven useful in bringing dying tissues back to life, among a variety of other benefits.
Props to you, ancient doctors, for going with the creepiest treatment possible and being proven right.