I've certainly seen requests for sources used obnoxiously by people who felt zero need to back up their side of the argument with sources. But it's also often a legitimate request to see what the OP is talking about.

The internet is vast. I sometimes have difficulty finding things I personally saw before and know exist. Slightly different search terms can turn up drastically different results and, with five million visitors per month to HN, it is a melting pot of people with vastly different backgrounds.