Backing is important and every so often you should replace it. Just place enough backing on the reel so your fly line almost totally fills the reel. Too much backing and the fly line will hit the foot section of the reel. For most trout fishing it does just fill the reel, but, it keeps your line stored in a large a circle as possible on the reel, hence decreasing the coil memory of the line, thus you get better casting.

Larry ---sagefisher---