When bloodletting was a popular treatment, it was (I've read) used only for conditions which were attributed -- rightly or wrongly -- to too much blood, e.g., fever.
The modern use of leeches I've heard of (unfortunately with pictures) is to clean up wounds. It was a few years ago that I saw the article in the Boston Globe -- I forget whether it's just in the case of surgical wounds to promote healing or other wounds. (I'm afraid the pictures made the deepest impression -- anyone who puts a leech on me without being sure I've been sedated and tied up will have his own wounds to worry about!)