A few reasons you'd want to use it.
Basically a buffer keeps one circuit from interfering with another circuit. This eliminates cross talk and electrical noise (which can cause a whole list of potential problems).
Also, sometimes one circuit might send voltages that could damage another circuit, and so a buffer keeps that from happening.
An optical buffer is like the ultimate type.
I've only needed a buffer like once or twice ever . . . I wouldn't worry about it unless you are doing highly sensitive electronics stuff.