Random Worldbuilding Questions (Biology, Geography, Society, etc.)

[hymer]

This being a harbour, things like grain, bars of metal and dried/salted/smoked meat is perfectly movable from there to civilized ports. None of these need be considered luxuries.
Rome for example was entirely dependent on grain supplies from Egypt for centuries.

Edit: But all of that is actually talking a bit besides the purpose. I was hoping to find some commodity which was not essential, nor a matter of luxury. Tar as was mentioned is a good example, as you can build ships without it, but your ships will last a lot longer if you tar them regularly (disregarding copper bottoms as an alternative for now).