If they are there and match the last eight characters of your VIN, the engine and/or trans are probably original to the car. I've heard of people restamping the characters to fake it, but that seems to be something done on high dollar B and E-bodies.
To some these numbers mean a lot, and to others nothing at all. But I think I've heard they are worth a 10-15% bump in the price of the car. It's up to the buyer if it's worth it to him or not.