When I invented tagging it was as personal curation process, it was designed for people to recall things back to themselves later. It was an organizational schema. It has mostly disappeared.
What we have now is mostly tagging so OTHER people can find it. Which leads to all sorts of bad incentives.
Nobody really built collaborative tagging but it needs a bunch more support than hashtag-this and hashtag-that to really work. For example, we showed people tags that they have used before so they were gently pushed to reuse tags for more organizational cohesiveness.