It's usually used by newer users in identification questions, to clarify what's being identified. New users also don't necessarily know how to properly use tags, so they often use it where a more specific tag would do (they don't know to be as specific as possible). In the Help Center, it is posted that the identification scope covers diseases and pests, as well as plants:
- identification, diagnosis or management of plant diseases, pests, or weeds
So if you aren't sure of anything specific (like if it's a trees, shrubs, perennials, flowers question, etc.), this tag would specify that the identification question posted is asking about plants, rather than diseases or pests.
Now, once the question has an answer, it could be retagged, and the plant tag could be replaced by something more specific.