ok, so the main usage of emscripten is to compile C++ to javascript and then call them from the "normal" frontend javascript code? ie. most UI related frontend job will still be coded in javascript?

As to the question why, C++ is not only about runtime performance, it's a more mature language than javascript/typescript and there are powerful data structure/design patterns/algorithms that are missing from javascript. Of course I see it this way because I'm way more competent in C++ than javascript.