Sometimes it's people who feel like posting oldies/perennials/classics, and sometimes it's people running across a thing for the first time.

We've had to learn not to crack down on these too heavily because there's always an up-and-coming cohort of new users who've never seen them before, and HN should be a place to run across them. Once a year seems to be an acceptable maximum rate (https://news.ycombinator.com/newsfaq.html).