Here's a simple example that's currently very hard to do and requires all kinds of hacky and unsatisfying workarounds:

1. A 3d model, say of a statue in a museum

2. Add annotations to the model drawing attention to specific features (especially if the annotations are not just a single word or number)

If you want the annotations to be properly occluded by the model as you move the camera around, it's hard - you can't use HTML. If you do use HTML, you'll have to do complex calculations to make it match the correct place in the 3d scene, and it will always be a frame delayed, and occlusion is bad - usually just show or hide the entire HTML annotation based on the bounding box of the 3d model (I have seen better solutions but they took a ton of work).

So you could use 3d text, maybe SDF, but now you've created a entire text rendering system without accessibility or anything like that. Also, if you want anything more than very simple annotations (for example, videos, lists, select menus, whatever) you either have to reinvent them or fall back HTML.