Many of them were family members buying freedom for enslaved loved ones.
Same with white people up north. Many Quakers would buy black people for freedom. This way if the black folks ever went back south the could not be re slaved, Kinda like an owners title to a car sad but true..
There is slavery going on today, Great men like Reggie White (Minister of Defense) would spend a lot of his NFL money buying black Children out of slavery in Africa and putting them in good adoptive homes..