There are all sorts of numbers that can match, and did match when the car was new. The VIN tag on the dash, the fender tag, the motor and the trans, the radiator support, and in some instances the trunk lip can all have the VIN or parts of it recorded.
I think the original idea was that, if a car had all the numbers matching, it probably had been well-treated. A car in an accident might have had the radiator support replaced, and a car that was hot-rodded might have blown the original engine. Whatever the original idea, car collectors took up this idea that a car has greater value if all it's numbers still match.
So, short form, if you're a collector, it's a big deal, and in that case, especially in the car supposedly has a bunch of rare options or combos that could only be proved by the fender tag, I'd avoid it. I personally am a driver, and to me the rare original and the clone both drive the same, so if it was a car I wanted, and all other factors were equal, I'd buy it.
