Use when the question is about when it is appropriate to add or remove tags while editing a question.
Every question on a Stack site can have up to five tags designed to categorize it.
However, sometimes a user uses tags that are unrelated to the question, or that - according to the tag wiki - shouldn't be applied to that particular question. In that case, it may be appropriate to remove tags while editing.
On the other hand, sometimes a question may benefit from having a particular tag attached to it, which the original author may not have known about. In that case, a user with edit privilege may choose to add that particular tag.
As always, respect the intention of the original author when editing, and check the Meta site for the community consensus on editing and tagging practices.