What's the difference between istringstream, ostringstream and stringstream? / Why not use stringstream in every case?

Question

When would I use std::istringstream, std::ostringstream and std::stringstream and why shouldn\'t I just use std::stringstream

Answer 1

Why open a file for read/write access if you only need to read from it, for example?

What if multiple processes needed to read from the same file?