Imu is such a bum bro why do the yonko and hell even the navy exist like brother you can just control everyone
The reason slavery/Heavenly Tribute exists in One Piece: Souls. Let's be real: does the WG actually need money? They print it themselves, as far as we know. No, they want control.
The Heavenly Tribute isn't about making money; it's about making slaves. It's about pitting countries against themselves, and each other, so that they can't stand against the WG. Because of the Heavenly Tribute, slavery/crime is rampant in the One Piece world. Pirates are either poor people from WG nations, or poor people from non-WG-affiliated Nations, who have to do what they can to survive.
Their lives don't matter; Saturn already said the lives of humans can replaced.
IMO, it's Souls. Imu and the Gorosei's magic needs SOUL energy to function. This also explains the "Great Cleansing": it's more or less harvesting a shit ton of Souls at once for maximum profit.
We already know from Linlin that Souls are a real entity/currency in the One Piece world. IMO, Imu's magic/abilities utilize Souls, too, but to a higher level. Linlin had to ask if you would give her lifespan, or will serve her: Imu will straight up take your lifespan + force you to serve as a slave.
Imu kinda feels like Moria + Linlin right now.