Let's say the water is cold, the inlet and outlet shutoffs are closed, then the WH is turned on. The water expands, and if everything is working properly, the T&P valve will open to relieve the pressure from the hot, expanding water. No different from what would happen if you don't have an expansion tank, and all of your valves are working properly. No big reason to add one, but I suppose it won't hurt; less water to flow down and out to drain and then be able to solder the new connections. Don't know all of the codes, but one of the pros will chime in if it's an issue.