stringstream/libc++ specific optimization, has any sense?

I saw such code in some cross platform C++ project:

#if defined(_LIBCPP_VERSION)
    #define CONCAT(EXPR)   (std::stringstream() << EXPR).str()    
#else
    #define CONCAT(EXPR)   (static_cast<std::stringstream&>(std::stringstream() << EXPR)).str()
#endif

As I understand there is feature/extension in libc++ and operator<< return std::stringstream, instead of std::basic_ostream<char>, but why not use portable variant instead of this extension? What is benefit for libc++?