As long as equipped creature is legendary, it has menace
It already passively shapes up your hand, but it enables so many things from discard decks to madness/mayhem to cast-from-exile strategies that just want another payoff for playing their game
New game-changing abilities give different Pokmon types more play, while other items and support cards bring new elements into matches that need to be considered by players
In terms of design, Manaphy is inspired by marine life, featuring a blue, jellyfish-like body with large, expressive eyes that endear it to many fans
Coincidentally, trading has become a great way to meet people and act as a conversation starter, there's a good number of times where I'd kill an hour or two just conversing with a local or a stranger during a trade either about the trade or something random