Coins only have one use and that is for purposes of trading.
They also have to lowest base price amount of all products at 1.
Most people consider coins useless in terms of buying them off of traders because of the commission that comes with buying them.
However because of the long amount of time you can spend getting your people and bots to move large amounts when you want to make a big expensive trade of like 100 items it is more time effective to use coins as they require no moving.
Also you can slowly build up a big amount if you start off with high production in a specific resource and are running out of space to store it.