One of the oldest arguments around is the stretch/strength balance of a plastic line. Nylon/fluorocarbon lines are extruded and then stretched to orient the plastic molecules to increase the strength. The more you orient the fibers the more strength they will have, but the less elongation to break they will have. The optimum solution is a balance between the two, which will vary for use and fishing style. Also, the rated strength is from a slow laboratory test with special grips - the strength can be less for sudden spikes in tension or for knots. Oriented plastics may also have less knot strength. Orienting the fibers lines them up and strengthens the line along its axis. However, you still need a little strength to the sides to tolerate the side wall pressure generated by a knot. Once again, there are different way to balance this and knot strength rather than simple breaking strength is the real measure for fishing.
One more comment, you typically need some give in the system to prevent tension spikes from happening - either the leader/line has to stretch or the rod has to flex when a fish makes a sudden run. Making the line and rod both stiff can lead to the tension rising quickly and break offs. Once again it is a trade-off. Need to minimize give for better feel and maximize give to prevent break offs. Since everybody fishes differently this allows a lot of lines and tippet materials on the market!
