Comment by 7e
AM transmissions have a far greater range than FM transmissions, up to thousands of miles. This matters in a common emergency scenario, a nuclear bomb blast (or all out nuclear war), when EMPs will take out most electronics. Cars are Faraday cages and their electronics will likely survive an EMP. In this case, an AM radio in a car is likely the last surviving broadcast medium people will have access to.
This only works if the AM antenna is protected, inside the Faraday cage, next to the radio. Otherwise you're left with a fried front end.