Exactly what makes a MOPAR a "Matching Numbers" car?

I think that matching number means that the vin on the car in various places matches the block and the trans case, but the real question is, WHY DOES ANYBODY GIVE A DAMN??? 45 years ago having the Vin match the motor and trans was a good indication that the car was unmolested and still worked like Ma Mopar intended - you didn't have to deal with unknown parts in the drivetrain put together by god knows who. Flash forward 45 years and all it means is that the block and trans case are original to the car. Chances are they've all been rebuilt once, maybe more, and unless somebody saved the receipts and you know who did it was an upstanding builder, matching numbers tells you next to nothing