As I understand it: (1) calling an optimized library written in another language for performance or size -- say you have a physics engine component in your game, or a graph traversal algorithm, or whatever; (2) calling an existing library that is available in another language with no JS equivalent; and (3) running a full existing application of some sort that is written in another language and giving it a Web interface. Oh, and (4) sandboxing untrusted code, and coming up or already present (I'm not keeping close track) (5) portability, where you can either run it on the Web or on a truly cross-platform runtime (WasmTime I think?).

(And the downvotes are probably because your question could be interpreted as implying that Wasm is useless and people are wasting their time. Even though you're asking a totally fair question, it's easy to read the tone as negative and dismissive whether you meant it that way or not.)

I have only played around with it, but to me the possibility of using a language other than JS to write web apps is really exciting. I don't hate JavaScript by any means, I've used it professionally off and on for years, but it still frustrates me that for webapps you're basically forced to use it.

I brought it into openEtG to speed up the engine's RNG: https://github.com/serprex/openEtG/blob/master/src/rng.wat

RAD tools like Delphi for WebAssembly or Microsoft .NET WinForms is what springs to mind.

Basically plugins for the browser that everyone can agree on having, instead of complaining that they are proprietary blobs.

its been more than 5 years, and nothing useful so far

you can do "cool" stuff like running DOOM in your browser... but why?