Why did the Normans need castles?

It wasn't just protection though, or pure military reasons. Castles were set up to put a presence there, to establish trading relations and to get the new countries used to their presence. They also facilitated trade -- a castle had large kitchens and other needs that could best be fulfilled by trading with the local populace. Many of the castles the Normans built were not fully garrisoned all the time, but were there for times of need and to interact with the locals.