I've bought horses with buy-back clauses and have sold them with it. If you need to flip a horse fast and care about their well being, the buy-back clause really helps because the previous owner already showed interest before the horse changed ownership. Calling the previous owner is like already having a buyer interested before you go through the trouble of putting ads up. If the previous owner isn't interested, then maybe they know someone who is before you have to bring in strangers to check out the horse.

A buy-back clause is just an attempt at common curtesy.