Usage of std::stringstream over std::string without locales

I can't really understand when I need to use std::stringstream instead of std::string. I read that std::stringstream supports locales, but if I don't need them, I can just use std::string and that's all, right? If I run into a situation that std::string can't do what I need to do, I can see if std::stringstream can. In general, my concern is that std::stringstream was introduced after std::string, so maybe it does something better (faster, more memory efficient) than std::string, and in some cases I should use it instead of std::string?