Is this a legitimate question or a troll? The leader is where most of the force is when fighting fish, especially if you lose tightness and get sudden jolts. It's also the piece of line most likely to get caught on a rock and get snapped off. Heavy leader = less dropped fish.
When fishing for Giant Trevally, I use 60lb mainline and 100lb leader. When fishing for big Snapper, 20lb mainline and 40lb leader. This is pretty much the standard across most fisheries around the world. Obviously there are exceptions, like when you are trying to entice a bite on a picky fishery, so you size down your leader...but as far as fighting fish goes, having a heavier leader allows you to be much more aggressive. It should also be taken into consideration that when I talk about mainline it is strictly braid, which outperforms its test rating by quite a bit. Apart from the obvious fact that lighter leader = potentially more bites, I can't see any performance benefit from doing this.
Funnily enough this is something I have been asked 2-3 times since moving here. Not sure I quite understand the reasoning behind the thought process, maybe you can enlighten me?
We'll I believe the thought process is simple: when playing a fish, the force is exerted on the whole line, not just the end (leader). The weakest point of the line is going to be the weakest knot and the thinnest line. If your main line is lighter than your leader, and we are going to take into account your claim that your braid outperforms the rating to which it was tested(not sure how it manages to do that). Therefore, if your mainline is rated 10 pounds (but actually performs to 12lbs) and your leader is rated 20 pounds and the knots tying each are equal strength, and the fish on the hook applies 15 pounds of force then the main line is likely the one that will break (usually at the knot) whereby you lose your hook, 20 pound leader, and likely any other hardware attached below that mainline knot, such as pencil lead or an expensive float. The only reason I can see for using a heavier leader is when the mainline exceeds the weight stress needs but you are worried about the fish wearing out the leader (eg: pike)-