Nearly every nation in the One Piece world uses the Berry as its currency. This might be done so that Eiichiro Oda doesn't have to invent a new currency for every single island that the Straw Hat Pirates visit. This seems fine at first, but it raises the old question of whether a universal currency is economically ideal.
As different as the One Piece world is from the real world, it's possible that the way economics function in both of them is more similar than one might think. Thus, looking at why there's no universal currency in the real world should say a lot about how or why the concept functions the way it does in One Piece.
Starting with the positives, having a single currency between multiple nations would eliminate conversion costs. Normally, when people exchange their money for the currency of a country they're visiting, they have to pay an additional fee to have that money exchanged. A universal currency circumvents what would be a constant need to exchange one's money as well as the need to pay that conversion cost.
This is what makes the Berry so important in the One Piece world. This world is essentially one big archipelago where every island is its own nation; having to convert money at each island would be cumbersome and costly. The best options would be to limit traveling or rely on a universal currency.
A centralized currency could also encourage trading between nations that use the same kind of money, though this probably doesn't apply to One Piece. Trade is severely limited in this series due to the tumultuous nature of the open seas, which is also why the disparity between many nations' technological advancements is so great. If relying on the Berry mitigates this issue, it's not by much.
The Berry also seems to function well as a stable currency, and its value has remained consistent throughout the series, with no significant signs of depreciation or appreciation. This reliable central currency can be used as the basis for economic development in all countries that use it, including developing ones.
Unfortunately, there are some potential consequences to having a super currency that may very well exist somewhere in the One Piece world. For one thing, such a system would mean that any nation facing internal complications like war or natural disasters can't change its monetary policy to adapt to them. They would instead have to rely on other nations with more stable economies to help out.
If, for example, Wano relied on the Berry but was destabilized from being overrun by pirates for the last 20 years, they couldn't alter the Berry's value to fit their economic needs. They would need other Berry-dependent nations to provide aid so that the economic collapse doesn't spread to their countries. In this same regard, developed nations around the One Piece world are likely negatively affected by the sufferings of less fortunate nations.
Independent currencies allow each nation to enact aggressive monetary policies to fit their own needs. Luckily, Wano is one of the few nations in One Piece to rely on a separate currency based on gold, silver, and platinum coins. It can adjust the value of its own currency to fit its own needs.
The issue of not helping underdeveloped nations would be further aggravated by another problem with centralized currency, a central banking authority. This supranational entity couldn't prioritize the needs of an individual nation as that would create political bias. It would have to implement policies that treat all nations the same, regardless of their economic circumstances.
In the world of One Piece, this issue of equalized monetary policies can be observed through the Heavenly Tribute. These taxes on the member kingdoms of the World Government, while insignificant to wealthier nations, can leave poorer countries suffering the risk of famine. Regardless of any one member nation's inability to afford this tribute, they're all obligated to pay it.
In the real world, a universal currency is highly impractical; the cons of such a system outweigh the pros by far too much. It's instead better for different nations or groups of nations to have their own currencies with fixed exchange rates. As for how such a system would function in the One Piece world, that might require individual nations to split off from the World Government and create their own currencies. Considering the series' overarching theme of freedom, this solution may very well be realized.
For now, however, the Berry seems to work just fine as the main currency of One Piece. It keeps the economic side of the story neat and tidy while also potentially providing another minor conflict that will need to be overcome. It doesn't really have to be explored to keep the story engaging, but it's definitely something to think about.