Yeah I don't think anyone would really choose to write a new web app with Qt. I assume the target market is people that want to port their existing Qt code to the web. It could also be useful for demoing Qt apps.
It makes sense, from the perspective of shops who invested heavily in upskilling on Qt, to reuse the knowledge to build web apps.
regarding the how.. Qt provides a CMake wrapper which sets up the relevant flags automatically so with CMake/Qt projects it pretty much just works. Here's my build scripts:
- https://github.com/ossia/score/blob/master/ci/wasm.deps.sh
- https://github.com/ossia/score/blob/master/ci/wasm.build.sh