Why do people keep adding "obvious" IDs like "Plants" or "Fungi"?

This usually happens when you did not add an ID yourself, so your observation is sitting in uncategorized limbo. 

By adding a coarse ID of "Plants" or "Fungi" these identifiers are making it easier for other people to find your observation. Many experts use our taxonomic filters to focus on their taxon of expertise, so if you post an observation of a plant with no identification, these experts will never find it.
People adding coarse IDs are almost always trying to help you get more specific IDs.