Contains spoilers for One Piece chapter #1055
In a One Piece world dominated by over-the-top characters, one of the essential individuals in the story has always been Kaido's right-hand man Jack the Drought. While he may seem mundane, Drought's importance exposes a key trait that any pirate crew hoping to capture Gol D. Roger's treasure must have.
Like any other Emperor of the Sea, Kaido's rise to the top of the One World pirate community was largely due to his individual drive, power, and capabilities. It also helped that an opportunity presented itself, and Kaido capitalized on the chance. It is equally true that no one becomes an "emperor" without a little help from their friends, and Kaido had plenty. In terms of power, Jack had superior levels, with his abilities improved after eating the Zou Zou no Mi Devil Fruit. Yet even if he paled in comparison to pirates of similar status like Zoro, Sanji, Nico Robin, Trafalgar D. Water Law, and even Brook, Jack had one ability that proved absolutely essential to Kaido's rise. He is a fish-man who can swim the seas of the world with abandon. A skill that only grew after eating the Devil Fruit, maintaining his ability to breathe underwater.
As a result of Jack being a Fish-Man, his half-man/half-fish body allows him to breathe air on land like a human, as well as breathe in water through his gills in water just like every other fish. While this ability may not seem useful to people who spend most of their lives on the water, Jack's abilities were fundamental to surviving in the water.
Prior to eating the Devil Fruit, Jack could literary jump into the water and help the Beast Pirates navigate especially hazardous areas of the sea. Jack's usefulness was most clearly illustrated in One Piece Chapter 1055, shown to be the vital element in Kaido's understanding of all that was submerged in Wano. Indeed, since no other crewmate could have swum to such depth either as a result of eating a Devil Fruit, or due to the fact that they were physically unable to reach such a depth or unwilling to attempt it, Jack was clearly the only individual on the crew who could have reached the underwater cities of Wano and read the Puneglyph that was found within them.
The availability and accessibility of Jack to Kaido and the Beast Pirates for waterborne activities should not be overlooked. Despite many of the most important One Piece adventures happening on the sea, many of the world's most powerful beings are limited by their inability to operate in water. As such, characters such as Jack the Drought provided essential water-based services without which would have stopped many of the most important voyages and discoveries along the Grand Line.
Look for One Piece's newest chapter when it releases on Viz Media's website.