I use tagging for sorting media that falls into multiple categories and defies normal hierarchical methods. I didn’t know about tagging users on Lemmy though, but I don’t know that I honestly will use it ever. You’re all just random people talking into the void, hell, half (or more) of the people here may very well be bots, I don’t know. The idea of tagging people based on past interactions seems a bit weird to me I guess (not that I don’t see uses for tagging for organization in general).
Edit: actually, I don’t even know if user tags are supported by Voyager, the main app I use to access lemmy. So, that too.