I've found accounts (possibly bots) that seem to go find the accompanying HN thread, and post a comment from it onto the reddit thread for karma. I sent reddit's anti-evil team a note about it since it's probably karma farming, but they never responded. Maybe it doesn't matter? It's not like we hold exclusive rights to stuff on here, so there's no legal issue, it just seems to be an efficient way of getting karma for that specific subreddit.
The reposts also result in a lot of not programming related content getting on that sub, which none of the mods seem to delete very often (stuff that should go to r/sysadmin or even r/technology).
I used to do something like this any time I detected twin threads (discussing the same URL) across HN and reddit. If anyone asked any unanswered question at one source, one of my bots would ask it at the other sources (plus Quora, usually) and wait for a response elsewhere, then paste that response back to the original OP with a link/citations. Including the citations got me autobanned a few times (which reddit admins graciously removed, repeatedly); if I weren't concerned with plagiarism, bot management would probably have been much smoother.
Would be pretty trivial to skip all the question/answer stuff and just share comments around sites. In a vacuum, I'd say it could be argued that mirroring comments around the Internet would result in good in various ways (sharing information, letting people choose what site they want to use, limiting censorship and/or site downtime, getting answers to people who might not know the best place to ask them, etc).
What you saw was probably karma farming, but could also have been someone trying to help in some abstract way. :)
The reposts also result in a lot of not programming related content getting on that sub, which none of the mods seem to delete very often (stuff that should go to r/sysadmin or even r/technology).