Knot-less tapered leaders are extruded from a machine to create a seamless length of nylon that tapers from the butt to the tippet. One advantage of having a single piece of leader material is there are no knot connections to fail. Plus, after you've changed flies a few times, you can simply tie on a piece of tippet to the end of the leader to make up for whatever you've clipped off. The simplicity becomes a trade-off, however, since you don't know exactly where the butt section ends and the tippet starts. Once you begin cutting the leader back, how can you be sure you're maintaining the proper taper?