They're one of the original 'tribal' creature types: fish folk has been a viable strategy since the very earliest days of Magic: The Gathering
We've had cards that care about basic Energy before, like Perrserker and Raihan, but being able to put them directly onto your benched Pokemon is powerful
Characters related to the sets main theme, Paramount War, are all here
Master Duel, for the most part, dont focus on one particular archetype, these builds tend to mix and match and use destruction power to synergize and bring out powerful Monsters
They are points that may not be obvious for someone not familiar with these products, like many of the points already mentioned, but clear signs for someone who knows what the actual products look like
Any way to distinguish on the outside of the starter gift box which 2 player starter set is included inside