If there weren’t a system agreed and used by the majority of everyone, then there wouldn’t be a good way for items we want to need to be gained. Of course, there’s always trading items for other items, however, it’s not like those types of trades are the most convenient, especially if you’re trying to get something like a whole sack of rice for another food item or a created item. It’s not convenient, so having a set type of currency is important to have.