[capnq]

I'm pretty new to Dom3 myself, but most of my limited experience with Dom2 still applies, so here are a few thoughts.

I most often build fortresses to protect provinces where I can recruit useful units that I normally don't have access to, or that are so expensive that the province can't even recruit one in one turn without a fort. For example, in one of my solo games, where I'm playing EA Marverni, I've built fortresses in the province where I found the Azure Academy (W2 mages), and a province where I can recruit heavy cavalry. (Marverni already has fairly heavy cavalry, but their Eponi Lords don't use lances.)

I have never needed to build a fortress somewhere for its supplies.