It wasn't what one might think: he was a priest.
Also, a notorious Cardinal vouched for him.
Seriously, few souls are more lost than those who find themselves in situations where they sell themselves for money or whatever. They are in a dreadful trap and society does not always make it easy for them to get out of it.
Father John disregarded the cynicism of society and reached out to these lost sheep, establishing a place of refuge to which women of "ill-fame" could go and change their lives.
He did other good work as well, from educating the clergy to spreading devotion to the Sacred Heart. He also established new religious orders (for which he needed the political assistance of Cardinal Richelieu and others).
St. John Eudes died at the age of 78 on this very day in 1680.
(from an earlier post)