I have a few reasons.. one i never have to worry about abrasion on my main line, if it is dragged across logs or rock it doesnt get beat up, when running light line and you happen to damage that line say 30 ft or more from the end you pretty much have to replace all that line or take your chances of it breaking at that point with a fish on. it is much easier to just change out my leader which never gives out on me and is virtualy invisible.

second i fish mostly bait casting reels and the heavier line is much easier to cast and work with in the case of a tangle or rats nest.

The leader it thicker then the braid im using and provides exactly what i need it to.. invisibility, im not worried about losing fish with it at all.

How many people say using light line is the best thing ever, but then tie on a steel leader? Thats the opposite of what i do, much better strength but you might as well use heavy line then cuz your leader is clear as day down there..