The final paragraph of Swift's "A Modest Proposal," first, serves the usual purpose of a final paragraph in an essay by providing the conclusion. He summarizes the benefits of his proposal, for instance:
- it's for the public good
- will advance trade
- will provide for infants
- will relieve the poor of their poverty
- will give some pleasure to the rich
Beyond that, the final paragraph establishes that the speaker is not making this proposal so that he can profit financially from it. He has no children that are not already too old, and his wife is past child-bearing age.
In general, in addition to these specifics, the paragraph contributes to the irony of the piece, as well as the humor. The speaker presents his proof that he is not trying to get rich off of his proposal as if it is a serious proposal, continuing the irony used throughout the piece.
In "A Modest Proposal," Swift develops his argument through satire. Swift's purpose is to criticize the English response to the severe poverty in Ireland that occurred as a result of the draught in the early 1700s. Throughout the essay, Swift has employed rhetorical devices that are commonly used in satire such as irony. In the final paragraph, Swift uses litotes ("I have not the least personal interest"), a form of understatement, which on the surface adds to the humor of the piece but at the heart of the essay really suggests that Swift is greatly concerned by the plight of the Irish and the response of the landowners.
In the final lines of the paragraph, Swift continues his use of the absurd to relate back to the absurd scenario that he has created in the satire. The situation metaphorically represents Swift's critique of English landowners, and serves as the final point on which the reader should linger.
I think that the first answer does a good job of summing up the main points of the final paragraph. However, I do not think it really addresses the rhetorical purpose of that paragraph. I assume that's why you have rated it poorly, at least...
To me, the importance of this paragraph is that it really provides a good finish to the tone of the essay. The whole essay has been written in a very serious way. Swift is pretending that he is making a serious proposal.
Because of this, it makes sense for him to finish off the essay in a very serious way -- to make it sound to the very end like he really means what he has said. So when he finishes off by saying essentially "look -- this is a really good idea and I'm not just putting it forward to help me get rich" he is ending the essay in just the same tone that he has presented the rest of the argument. This makes it, to me, a very effective ending.