Why does the author of "lifeboat ethics" choose to begin his essay with this lifeboat metaphor?

It is because he wants to draw a parallel between a lifeboat in the water with 50 people in it and 100's in the water with room for only 10 more to the plight of poor nations compared to the few rich nations and the problem of distribution of wealth and the questions that arise like do I take on board someone who is nearly dying in the water or someone who is more robust and do I allow cannibalism after a person has died, for example?

Day 1: separated light from darkness day 2: skyday 3: land and waterday 4: sun,moon and starsday 5: creatureday 6: man and humanday 7: rested