But the native version already does that! What can electron do that the native version cannot ? The point of electron is that it let you write code once (in JavaScript or something that transpile to js, like TypeScript or Elm) and built multi-plateform software. But if you have a native software that can be built in wasm, it also can be built on Windows + Mac + Linux. Why using electron in that case?

Electron is great if you don't want to write native code (and deal with the debugging complexity of C or C++). If you already have a cross-plateform native app in written with Qt, why the hell would you want to put that in Electron?! And remind that if your C++ isn't cross-plateform already, you won't be able to build to wasm.