Years ago, what did people use leeches for?

Years ago, leeches were used to draw blood.... what was believed to be bad blood.

"The Good Lord, yes," agreed Mrs. Tennyson, "but not Dr. Hawthorn. That quack wanted to put leeches on her stomach! Leeches! My word! He said they would suck out the bad blood. Now you tell me. How would a leech know good blood from bad blood?"