Even if the story doesn't make much sense, I enjoyed fighting my way through World of Light. It's a more compelling way to acquire new characters than waiting for a new challenger to appear as you fight in other modes. The matches are also more interesting, since your opponents also have spirits of their own. That means you could end up fighting a giant Donkey Kong, an Isabelle (from Animal Crossing) that's constantly healing, or an army of Mr. Game & Watch. Some matches are genuinely difficult, and similar to an RPG, you might have to wait for your array of spirits to get stronger before you can emerge victorious.
You can also rescue support characters from the Spirit Board mode, which basically offers you direct access to the fights you'd see in World of Light without the world map. There are a few twists, though: The spirit battles are only available for a certain time, so if you really want a specific character, you're pretty much forced to fight as soon as it appears. There are also a variety of difficulty levels for every battle, which are clearly labeled before you jump in. Once you win the fight, you also have to free the spirit from its prison with a well-placed blaster shot. If you miss, the fight goes away and you'll have to wait for that spirit to reappear.
It's not surprising that Nintendo is investing more in the single player experience, since it gives you something to do with the game when you're away from friends and the internet. After spending a few hours in World of Light, I've only made a slight dent on the map, so I'd wager it could easily last dozens of hours. The Spirit Board, meanwhile, offers a quick hit of action for players on the go.