For me it's portability first, decent performance second and third the 'seamless distribution model' of the web (no installation, no app shops, no code signing, just an URL).
I wouldn't write a web-only app in C++, but if the same code needs to run on other platforms as well (e.g. native mobile, desktop or gaming consoles) then C/C++ is basically the only choice. Also, at least in emscripten, compiled code is faster then hand-written JS for several reasons (asm.js, LLVM optimizer passes, no gc), so it makes sense to write stuff like physics/pathfinding/AI engines in C/C++ and compile it to JS.
Plus, it's fun to write a desktop OpenGL demo, flip a build system switch and run it in the browser ;)
I wouldn't write a web-only app in C++, but if the same code needs to run on other platforms as well (e.g. native mobile, desktop or gaming consoles) then C/C++ is basically the only choice. Also, at least in emscripten, compiled code is faster then hand-written JS for several reasons (asm.js, LLVM optimizer passes, no gc), so it makes sense to write stuff like physics/pathfinding/AI engines in C/C++ and compile it to JS.
Plus, it's fun to write a desktop OpenGL demo, flip a build system switch and run it in the browser ;)