Compatibility. Code written for layer 4 should be compatible even if layer 3 is changed. In practice much code had to be changed anyways, but the amount of vi

Compatibility. Code written for layer 4 should be compatible even if layer 3 is changed. In practice much code had to be changed anyways, but the amount of violence applied to existing code was limited by not getting rid of ports. It's not just that either, but much else, including cognitive load.