One of the great joys of travel in Western Europe is that you can plan as much or as little as you want. Some people will have a must-see list of sights they will feel cheated if they miss. Others will go with the flow and let their moods and experiences shape their journey. Neither is the better way to go and many people will combine aspects of both. They'll have a realistic idea of what they can accomplish while still having the freedom to take a detour on a whim.
Planning also helps you figure out how much you'll spend and when is the best time to travel for the sort of trip you want.