Bartering also assumes, for your example, that the apple farmer wants oranges and the orange farmer wants apples. Consider that either one may not. And then consider how many other goods exist even in primitive agrarian society that people may or may not want at any given time.

Currency is whatever the agreed vehicle is for value exchange that solves this. The apple farmer can now sell apples for silver nuggets and use the silver to buy tools from someone else later, which the orange farmer either doesn’t have or is unwilling to part with. It allows for more complex transactions that then helped grow more complex societies.