Way back in the old days, copper plates were engraved by craftsmen, inked over and pressed onto paper in order to produce prints and illustrations.
Copper is a soft metal and with the pressure of each pressing, the plate is flattened ever so slightly.
This meant that there were only so many prints that could be pressed from the plate, thus every print by it's very nature was a limited edition as only so many could be produced.
Fasst forward to the 20th century and things change, this is the era of photomechanical production of prints and now of course publishing from digital images.
... meaning that any number of prints can be produced, each one exactly the same as the one before and theoretically any number could be produced ... basically unlimited numbers can be printed!
So publishers had to come up with a way to preserve value by limiting the print run - so the concept of the signed limited edition was born.