They effect how valuable the production is in a province wine makes more than grain so on and so forth. The value of the good is also added to to the trade node to create its local wealth they are what you are trading in with in the node, so difrent trade goods make some provinces more valuable. Unfortunatly i dont think there is a trade good map mode to give a quick view of whats where, I had a look last night and couldnt find one which is annoying.