If you ran out of all resources in the game you could still continue as long as you have coins due to the first supply building being built via coins. Both are main resources yes, but coins are the foundation that allows everything else to be produced. By building coins into the town hall the game ensures that no matter how reckless your resource management you can never be in a position where it's impossible to recover. It's a failsafe against the worst case scenario
The biggest difference between coins and supplies is once you have enough that you could get away with never producing those starting resources again the coins still have a use through buying Forge Points. Supplies have very limited use on their own and the things you could buy with just supplies or supplies + coins are generally either a small cost or a once off cost.
As a side note though it's actually possible now to make the town hall produce supplies. 3rd Viking Emissary which you get from the 7th Viking Cultural Settlement