X marks the spot. Why do they say that? Is it because on the old treasure maps they would put an X to show where the treasure was buried rather than writing "here there be treasure". It was obviously something they didn't trust to memory or shipmates. Were there back-up treasure maps? Or furphies which put the X somewhere which wasn't the spot where the treasure was? And why was it an X and not a Y or even a number? Maybe it had to do with map references, the intersect of longitude and latitude lines. They wouldn't have had triangulation then - or maybe they did and knew how to use it - although it seems a little more difficult than specific map coordinates. Of course, it's hard to know if pirates really did bury their treasure - it probably wasn't something that was publicised. It definitely was the stuff of fiction though with the pirates of Treasure Island having buried treasure aplenty but research suggests it's usually the stuff of legends rather than real life.
But even if X didn't find a place on treasure maps, it did prove useful - and still does - how could you say 10 in Roman Numerals if X didn't mark the spot?