Two reasons: First, everything has context and time is often a big part of it. Setting that context up for people is helpful. Second, this site is entitled Ha

Two reasons: First, everything has context and time is often a big part of it. Setting that context up for people is helpful. Second, this site is entitled Hacker News and this is not News. While people will argue old things should still be included, and that's fine, I think old things should at least be marked as such by putting the year of publication next to them.