What is the purpose of the last paragraph of A Modest Proposal?a modest proposal
The whole point of Jonathan Swift's "A Modest Proposal" is to satirize the British rule of Ireland.
Swift's proposal for the problems that British rule has (in his opinion) brought to Ireland is for the Irish to sell their children to be butchered and used as food for the rich of the kingdom.
This is, of course, a horrible and disgusting idea. But the brilliance of "A Modest Proposal" is in the very straight-forward and matter of fact way that Swift presents this proposal. He does not acknowledge the fact that it is so horrible.
The last paragraph is meant to bolster this impression that the essay is very serious. He asserts that the plan will not benefit him personally and says that is proof that he is doing it for the good of the public and not out of self-interest.
So the purpose of the paragraph is to wrap up the essay by underlining Swift's fake serious tone, which is what makes the essay so powerful.