Choosing the right leader and tippet can be the difference between a successful fishing trip and a less successful one. When fishing for shy fish, long and thin leaders are often required, preferably with tippet fluorocarbon material. Fluorocarbon is stronger than regular nylontippet in the corresponding dimensions.
But sometimes you don't want too thin leaders completely unnecessarily and instead lose the fish due to a leader break. One such example is when fishing for salmon and sea trout, these are less sensitive to slightly coarser leaders , it is therefore preferable to have a slightly coarser dimension to avoid the anxiety that creeps in when you have lost the salmon of your dreams due to a leader crime.