>>542728In modern hospitals it's pretty much only used in plastic/reconstructive surgery.
You see, when they do certain types of skin grafts, they take an "island" of flesh along with its arterial blood supply and move it as a whole somewhere else (careful to never sever the arteries). This is important when grafting areas with poor vasculature or in large grafts that may die off before it establishes a new blood supply.
The problem with this is, nobody really gives a shit about the venous supply. But that's okay because the body will usually make them and it's not mission critical to have them. Sometimes it takes longer than usual for the venous supply to figure itself out so you have to take out the blood going in through some method.
In come the leeches.
>final year med student here>note the blue-ish mottled appearance of the grafted region, this indicates that there is blood collecting in the region and it is become de-oxygenated 