A former slave could make a more persuasive messenger for abolition than a person who was never enslaved by appealing to the audience's emotions, to a former slave it would be easier to awake the empaty of the audience because he will transmit the accurate feelings that would appeal to the audience's fears, desires, etc.
Another way to make a former slave a more persuasive messenger for abolition is to present the evidence that demonstrates he was a slave and to use his anecdotes to convey a touching message.