Southey's Abolitionist Poetry

In "The Sailor who had Served in the Slave-Trade" Southey regales his with the tale of an nonconsensual (in terms of the slavery) sailor who had worked the slave trade. The preface to the poem provides grounding in Southey's following stanzas; merely by stating that this poor bloke was found wailing in a cow house when a minister chanced upon him, essentially a very Christian, "Be wary and take heed of this poor sailor's woes, or else it might happen to you as well!" I always find prefaces in poems bewildering... This preface, I find equal parts amusing and perplexing. Amusing, due to its concise nature in its telling and perplexing as to why Southey felt it necessary to include it. The sailor's story was one of agony, but was it presented "without the slightest addition or alteration?" I think not; a sailor (unless inebriated) does not wail in a cow house and then tell a minister the horrors he's seen in a rhyme scheme. Was Southey attempting to give credence to his work because he hadn't experience the slave trade himself, or was he attempting to gain a wider audience with this "factual" account? Probably both.