To be used when you know, or feel like, there are more tags to be added but don't know, or can't remember/find them. Tagme is automatically removed once the target post has reached 30 tags.
More specifically, it should be kept until an image can be reasonably determined to be tagged to at least Level 1 per the howto:tag checklist.
Not to be used as an excuse for laziness; tag your uploads as best as you can.
