It's a bit like theater - left and right are different depending on whether you're looking at the stage (such as from the audience) or looking out from the stage. So you have to have "stage left" and "house left" (and the same for right) to distinguish them. But it's still confusing if somebody just says left. Which left?

So they could just say something like "ship left" and "ship right" that are always referenced to the bow of the ship, and that would work almost as well, but having special terms just removes any possibility for confusion.