Earlier quoted context omitted.
A sailor, looking towards the back of the boat: - There's an incoming boat ! - From where ? - From the left ! Is it the left of the sailor, or is it the left of the vessel ?
You only know which side is port or starboard by figuring out your orientation compared to the ship and picking the left side of the ship. So it’s not actually achieving anything. You could just say left always refers to the ship’s left.
What you achieve is a new set of coordinates relative to the vessel. You are going north, sitting on the port, with a glass of wine in your right hand, instead of going up and sitting on the left.