There's more than that: I've also created mtg-double-faced-cards, mtg-competitive-rel, mtg-professional-rel, mtg-regular-rel, mtg-color-identity, and mtg-emblems.
The rationale is that our MTG tag hierarchy has been super undeveloped (borderline nonexistent) which makes it extremely difficult to find and reference past answers or find past duplicates. Very often I simply cannot find those questions at all even when I know they're there to be found. Our canonical questions on a great many topics are lost in a giant uncategorised pile of questions tagged only magic-the-gathering.
We have a curation tool for this scenario—tags—which has been underutilised and which I'm now applying to help us resolve this problem. There are many ways to slice up the metaphorical tag pie, so what I've been doing is working out distinct, significant, and unusual topics where we see questions very specifically about only that topic and where there is a lot of distinct confusion about the mechanic or topic itself.
The most concrete case where this is valuable is when this helps us find and make duplicates. You highlight mtg-indestructible as not having value, but in the process of adding that tag we found 3 never-before-closed duplicates in only 14 questions:
These duplicates were never identified in the beginning because it was extremely difficult to find questions about the indestructible mechanic. (You say keyword searches should pick them up, but I'll get to that shortly.) Now it is trivially easy to find questions about the indestructible mechanic, and when we see future questions about it, we can check for duplicates again then too. So they are already giving value.
Similarly when I added mtg-double-faced-cards we found these to be duplicates:
I think it's noteworthy these questions are the only duplicates in those tags. We didn't just find new duplicates—rather, before consolidating the topics under tags, duplicates were simply not getting discovered and closed at all.
There's other ways these tags add value to the site:
- We gather topics together to help bring them attention to the focus of experts who they need to reach. Because we have mtg-competitive-rel, mtg-professional-rel, and mtg-regular-rel, people who are experts on Rules Enforcement Levels can watch these tags to have them highlighted. (And indeed each of these tags has one watcher at time of writing this.)
- We gather topics together for newbies to learn about. If you've just visited our site confused about the commander format, color identity, regeneration, indestructibility, planeswalkers, emblems, double-faced cards, or rules enforcement levels, you can dive into previous questions on that topic to learn other things you might have been unclear on.
- We make it easier to find prior duplicates.
- We make it easier to find and reference canonical questions and answers on a topic.
- We make these topics more easily searchable and discoverable because they're actually not by keywords alone.
There are so few questions on these topics, there's no point in creating a feed for these topics, nor do they help with searching as general keyword searches would pick them up anyway.
Not actually true. When I run a search for
[magic-the-gathering] indestructible is:question I find 55 questions. Despite that, there are only actually 14 questions directly about the indestructible mechanic, now tagged as such.
75% of those keyword results are noise!
This is because most questions showing up in search are proposing a scenario in which indestructibility just tangentially comes up, but the questions are not, themselves, about the topic of indestructibility.
The mtg-indestructible tag lets me cut through the noise to find the questions fundamentally about the mechanic.
I don't want to look like I'm cherry-picking though so I'll examine the other two tags you listed as well:
These noise levels are why we weren't finding duplicates. It makes the topic difficult to dive into because by keyword alone we're finding tons of things that are not actually about the topic but are just tangentially mentioning it.
These noise levels aren't true of all of these tags. There's only 10 MTG questions that mention emblems and four tagged mtg-emblems. It's not hard to sift through those. Same goes for mtg-color-identity. But these are giving us value by gathering together a topic of focus and making it trivially easy for newbies and experts alike to find questions on that topic.
There's also usefulness when a tag isn't easily keyword searchable. mtg-double-faced-cards has a lot of equivalent keywords: do you search transform or double-faced or dfc or dual-faced or back face (catches a lot of questions about morph) or flip (a common misnomer that also catches flip cards)? Have you missed any keywords? Having a tag categorising the topic resolves this set of problems.