This could apply to many situations, but here's just one:
Say Alice asks a question on SO saying "how do I do X?". Bob then answers the question with "use library Y", and provides a link to the Y page.
Fast-forward several months later, where library Y is outdated and no longer maintained, and the official site that Bob linked to is 404'd. Carol needs to do X, but can't use library Y like Bob's answer suggested, but she can't post to SO because it'll be closed as a exact duplicate of Alice's question.