Comment by graemep

Because you can easily and cheaply convert USD to any other currency, and because it is the usual currency for international trade you can use it to pay someone else.

Suppose a British company imports tea from Sri Lanka and Kenya, blends and packages it, and exports it to retail chains in multiple countries. If all the buyers pay in the same currency used to pay the suppliers the British company does not have to convert more to GBP than required to meet its costs (and profits!) so loses less on converting currency at all. The usual currency used for this is USD.

So rather than:

customers currencies -> GBP -> suppliers currencies

we have customers currencies -> USD -> suppliers currencies.

Edit: I have not explained very well in the bit immediately above. The point is that the British company will not need to convert the currency at all as they will be paid in USD and will pay in USD. In most countries you get better rates converting to USD, and its easier to hedge this way. Even more so if there is a longer supply chain as then you get

A's currency -> B's currency -> C's currency to D's currency

vs

A's currency -> USD -> D's currency.

There are a few wrinkles on this in that customers may already have USD accounts, and suppliers might keep some money in USD, but obviously customers will be paid by their customers in their own currency, and will pay their staff and most other costs in their own currency.