In times past valve seats were features machined directly into the cylinder head or block. As long as compression was low enough and/or the octane was high enough there was no problems with the valves, in most cases, for the life of the car. The problem comes with higher compression and, especially, unleaded gas. Lead was used to raise octane numbers but it also cooled and lubricated valves, not an issue with intake valves which are cooled by incoming fuel, but a lifesaver for exhaust valves. Without lead the seat overheats making it susceptible to misshaping (think of a piece of carbon getting stuck to the valve and hammering the seat) and getting soft resulting in faster wear. To compensate for this, separate hardened valve seats were invented, that is a valve seat that is a separate part from the cylinder head or block and the part that can fall out, regardless of whether or not it is an intake or exhaust valve. Pinning is just a way to keep that from happening.
As an aside, my first, and for a long time only, car is a 1956 Ford Ranch Wagon. After I rebuilt the engine the valves lasted over 180,000 miles and needed replacing only after I could not find leaded gas anymore. Nick's (same guy that rebuilt the Escort's engine) fix was installing, you guessed it, hardened exhaust valve seats. The intakes were okay.