I would agree that there is no benefit to a person who comes from HN to read just that specific article. However, if I was a regular visitor to the guys blog and came across the article that way:

- Him posting the full text rather than a hyperlink saves me an extra click. - Maybe it also makes it easier to talk about the article with the rest of that blogs community. - It provides another copy in case the original goes down. - If he also has other posts it provides a consistent visual aesthetic across the body of information.

It's somewhat like creating a miniature library and discussion group.

I don't find any of these arguments convincing, but they're possibilities.