If you're inside a ship, it's often hard to say what way around you are: "left" and "right" have utility for the perspective of a person onboard. Port and starboard are the onboard equivalent of North/South/East/West when walking on land, especially if you're inside a building with no windows for external reference.
This is unlike for a doctor, where it's obvious which side is which: you can see the patient.
This is unlike for a doctor, where it's obvious which side is which: you can see the patient.