One for doors/trunk, one for ignition switch. Unfortunately, they had a very small number of variations, so the chance of being able to open or start someone else's car was good and car thieves knew this. Even with the small number of optional key cuts, thieves started peeling the column in order to start a car because they knew the police would arrest them for carrying tools for stealing cars when they had a full set if they weren't a locksmith.
When I did car audio, kids who just got their first car, often about 16-18 years old, would say "The square one is for the ignition, round (or oval, if it was a Ford) is for the door". Thanks, junior- I never would have figured that out.