I recently had my edit suggestion rejected when I tried to add a tag to someone else's post. I grant that my new tag didn't "help to define the topic of the question," (The body of the post itself does this quite well most of the time, and I confess I didn't even know this was a purpose of tagging.) but should I really not add an appropriate tag that would increase the visibility of a good question to people who can answer it? This seems odd but I trust there's a good reason for it.
For context, I tried to add differential-geometry to this post One parameter subgroup that leaves every compact set is a proper map