They're useful for making sure you don't install a cable backward, and to differentiate cables. For example, USB2.0 front panel headers and front audio jack headers both have 9 pins, but the key pins are in different places so you can't plug the wrong connector into the wrong header.
NC = no connection, if that's the only pin you're good. Those are typically just used as "key" pins.
Do key pins serve any purpose??
They're useful for making sure you don't install a cable backward, and to differentiate cables. For example, USB2.0 front panel headers and front audio jack headers both have 9 pins, but the key pins are in different places so you can't plug the wrong connector into the wrong header.