The most distinctive characteristic of a human is it's face - it is unique among each individual (with the exception of identical twins). It is uncertain to me if whether we best identify other humans by their face due to special brain parts developed for this (there are people with disease that cannot recognize faces) or for actual very distinctive physical face forms; maybe it is a combination of this.
Anyway, the question is about evolution. Why would distinct faces (or ability to recognize distinct faces) be something useful that would be retained by evolution? How is this better at survival?