If you are going on a hotel vacation, say WDW, and you arrive a day late, you lose a day of vacation. That is a little disappointing, but you get over it and enjoy the rest of the week.
If you are going on a cruise and you arrive one minute after the ship has left port: you are out of luck. There really is no rejoining the ship anymore. Your vacation has sailed away, taking with it all the money that you spent for food, lodging, and entertainment.
Flying in the night before adds $100 in food and hotel costs. Not only does it provide a stress-free environment, it gives you time to explore your port city or play in a hotel pool or check out the beach. This is vacation insurance that pays off more often than vacation insurance, and it even provides a little fun time.