We realized that 3D graphics in the browser were stupid and useless back in 1995...
...and slow! I was there when VRML was landing (just finished high school) and all I saw it used for were virtual rooms were avatars would talk in a 3D IRC like environment. Only big problem back then was we didn't have 3D acceleration and the interface was clunky and painfully slideshow like in speed.
As for uses, I could think of a few and have already started coding them. Instead of loading a PNG or GIF, it is pretty nice to be able to download a float array, be able to display it, and allow a user to interrogate it. Giving a user that kind of capability in the browser while not requiring them to download an application or a browser plugin is pretty darn nice. And while not mainstream, scientific fields could greatly benefit from something like that.