Well, at least in my area of specialty, tar is a major component of shipbuilding. Tar is often used as a shorthand for the whole industry of "naval stores," which are basically products derived from plant resin that are used in shipbuilding and maintenance. Tar in particular is used for sealing, caulking and waterproofing as ships. Tar in this context, derived from plants, shouldn't be confused from tar derived from petroleum or peat. I can't definitively say that this is the tar industry in your region without knowing the region, but tar and other stores were a major export of the Baltic and the Americas during the age of sail.

Traditionally, tar is used as a sealant for the purposes of waterproofing things. As far as I can recall, wooden ships had to be sealed against water by tar-soaked rope being wedged in between the wood. It is also mentioned in older medical texts as a way of sealing a wound, but I can't pull up the sources on that one at the moment, as I am at work.

As you can see, tar doesn't just come off the shelves, so a region that had access to tar pits would naturally make quite a bit of money.