If I buy it now, Ill be able to profit at a later date by selling to somebody else
By discarding "Atlantean Heavy Infantry", "Atlantean Marksman" or "Atlantean Dragoons", to activate the "Mermails" effects you can destroy a card on the field ("Heavy Infantry" and "Marksman") or search your Deck for any Sea Serpent monster ("Dragoons"), such as "Abyssmegalo", while also gaining the "Mermail" effects
Quick answer: The most expensive One Piece card is the Kaido [Championship 2023 Winner] ST04-003 at around SGD $48,280 raw a prize card awarded only to the 2023 World Championship winner
Second, some people just like to collect things
Among the "Dream Mirror" monsters, he has the highest ATK and DEF, so I think he's a trump card
Provided your deck has upwards of 38 lands (and closer to 40 is ideal, mixed with good draw engines) these one mana enchantments can help you drop an unfair amount of extra lands into play