If it had a date I would have said "OS on xkcd from a year ago - ah, seen that". If it doesn't have a date it must mean it's new and I _have to_ see it.

Yes, it sometimes help to see that it's something I've seen before, and many sites don't make the date very visible, so it's easy to miss that something presented as state of the art or news in the article isn't anymore. Having the year in the title helps to avoid that error.

In at least two situations, it helps.

One is when it's likely that I've seen the page before, but not so recently that I recognize it immediately. "XKCD #386 [2008]" (OK, I know that one by the number.)

The other is when the page has a particular viewpoint which is either explained by the date or unusually modern for that time period. "Tiny computers will be ubiquitous [1965]"