Daft_ishWhy is currency so essential?
BrikoXConsider what we used as currency before it was currency. You would have to barter before, which was inefficient. Common currency saves you and everyone involved time. Instead of having to barter for every item, which would also require you to do carry all of those items, you can just pay with currency now.
kevincoxA lot of cultures ended up with effective currencies. Whether that was grains of rice or chickens there ended up a small number of items that had a well understood value and ended up being the default item of trade, not because the receiver needed those items but because they were known to be easily exchanged with others.