I rarely use tags, opting mostly for custom fields, but I do use them as part of my performance reviews.
In our organisation, we have 8 behaviours that we have to demonstrate as part of our roles. I’ve set up a tag for each one. I have a private workspace, shared with my manager, and within that I have a different project for every one-to-one review. Each task is an example of a ‘highlight’ in my work, and I tag each one with the relevant behaviours. I’ve added all tags to my favourites, so I can access a list of every task associated with that tag, across all projects. Then, when it comes to my 6-month or annual review, I can pull out specific examples related to each behaviour.
I could accomplish the same thing with custom fields, but this just felt a bit simpler to set up.