Your villagers are the most important part of your village in Banished. Keeping them fed and warm in their homes is pretty much your top priority and where all of your resources go. Villagers will stock pile their homes with what they feel they are going to need and they will get those items from Storage Barns, Stock Piles (which I am lumping together for this discussion) and Markets.
As far as I can tell the market is pretty much the same as a storage barn and stock pile. All the items they can get from one they can find from the others. So I therefore ask what benefit is there to a market? If anything it seems that it may be able to hold more items but it is then limited to its coverage area while the storage barns and stock piles are not. While they look neat and employ people to just move goods around I just do not see why a market is needed.