Comfrey Leaves were introduced in DayZ 1.8. They are to be used by upcoming crafting recipes and currently serve no purpose.
As of 1.8.6 they are used to craft Sepsis Bandages, most likely an antibiotic bandage but this is yet to be tested.
After update 1.9.0 the comfrey leaves do not spawn anywhere on vanilla servers, and serve no purpose except being used to create a Sepsis bandage by combining a regular bandage with a comfrey leaf. The Sepsis bandage does not serve any other purpose than the regular bandage.