The fun is the main game. Those stages are like 1-2 hours of a 15 hour game.
But yeah if you have zero interest in a traditional 2d adventure game, I'd skip it. Even as an adventure game, the puzzles are very light and it's a lot closer to a VN presented as a lucasarts/sierra adventure game with full VA.
At the end of the day, 13 Sentinels is essentially a really good playable anime series. Either you're down with that or you're not. If you like sci-fi anime, then you will love this game. One of the best sci-fi stories in gaming since Mass Effect 1.