When you come across content that is less than stellar, it is important that the situation be found and corrected. That's not the role of moderators, but everyone on the site.
So why show me irrelevant posts?
When a question is closed, we don't want to start finding ways to sweep the situation under the rug. Community self-moderation is a big part of this site and everyone is responsible for its upkeep. If the existence of the question provided less than an awesome experience, it is up to you — someone who is looking for good information on the subject — to do something about it; to fix it or remove it.
There's a general philosophy that a closed question is not deleted. There is a small window where it may be on its way to deletion, but presumably it is still on the site because it contains useful information. For example, if it was closed as a duplicate, it may provide an entry point to find the post you wanted. But if the content has long since been deemed as unfitting, it should be reviewed and either improved or removed.
You don't want to be hiding content that is providing a bad experience. It's for everyone to find and improve, just like any other content — not leaving content festering in some unexplored corner of the site for the next person to find unhelpful.