They're supposed to be for whatever depiction you can find, really, but sometimes with tags it can get to a point where it might be necessary to differentiate if they are used often enough, like flower and stars.
Sanae is clearly very popular and is almost always depicted with her ornaments, even when she's wearing alternate, casual or adapted costumes. The only issue is these tags would only be used for her. You can still find images of non-hair ornament related frogs and snakes by just negating her from a search, but what if you want to find them in a picture with her in it? It's not an uncommon thing to for an artist to draw, either.
+1, I guess. Without nested tags there's no better way to handle this.