Comes in very handy for scrabble. It's the z in Menzies (which is pronounced 'mingis') and represents the ng sound. Written it's a z with a long tail like a 'g'. Also features in the surname Dalziel (pronounced 'dee ell' with the ng bit being silent).
Long ago I came to the conclusion that the Scots and the Irish can't spell. So they make up weird spellings to piss off the English. Which strategy has proven to be very successful. Not that the English can't do weird spellings too, they can, but the Scots are a whole nother ballgame.
I understand it all boils down to a basically celtic pronunciation, but written down by Norman French speakers using a very different alphabet which was not designed for the purpose.