Some (perhaps many) questions get asked over and over and we have an informal concept of a canonical-question. I've been trying to think of a way for people to find those more easily. I'm terrible at it, though that might just be me.
One of my thoughts was to have a tag for this, but my fear would be that it would be overused if anyone could apply it. If the concept of such a tag is useful at all, then the concept of tags that can be applied only by moderators might also be useful and possibly used for other things.
Another option, probably less desirable, would be a class of tags that could only be applied by high-reputation users. But, mod-only would probably work better since mods have back-channel communications open to them that others don't.
Maybe some other device could be found (or already exists) for canonical-questions that would make searching for them easier. And maybe more thought needs to be given to the general concept of mod-only tags.