It's related to bee borage, one of its folk names is "knitbone" - you use it as a compress (never internally 'cause it's poisonous).
Any herb with "officinale" or "officinalis" in its name has a long history of use in medicine, because that means that it appeared in one of the old herbals before Linnaeus got his classification system together.
The problem with comfrey, though, is getting it to STOP growing. It's an invasive weed to most folks, and it's best to grow it in pots or in a raised bed, and DON'T put it on the compost pile! 
