Some reasons that may induce the need for a valve job in a modern passenger car include: excessive RPM, high mileage, overheating, material failure, corrosion, and foreign object damage (FOD).
As the decades passed, however, engines ran cleaner and the addition of tetraethyllead in gasoline meant that such maintenance became less frequent.
Much softer yet aluminum cylinder heads, however, have replaceable pressed-in hardened steel valve seats.
Valves are inspected for damage such as cracking, pitting, channeling, and burning that will prevent proper operation and could lead to catastrophic failure.
Some older cars have relatively soft valve seats that wear faster when operated without leaded fuel.