Like others mentioned, trailing for anything that have high speed projectiles, most commonly used in pre-engineered plasma chargers or shards in AX, it helps with module sniping, but with MC's or anything with long range or focused that have travel time, will works here too. Reticle being gone at such setting, but it instead will show where projectiles will land. Lag have to taken into account, while using that mode, so better aim litte "ahead" where it shows to have best results. Fast ships that often change vectors, can make this mode "confusing", its kind of expert setting for general use, I suppose. Not advised to use with slow speed weapons - since "aim" can get out of view!
(unless you know whata ur doing, thats it)
Leading is more usefull with slower projectiles, such as PA's (875m/s), frags (667m/s) or pacifers (1000 m/s) and any kind of missiles (as it enables reticle), well as cannons.
Reticle also helps with ramming well as to visualise own vectors against opponents vectors, while module sniping with that setting can be tricky, its not as "confusing" as with trailing setting against nible targets while using slow speed weps, whenever such decide to suddenly change vectors.
It would be very nice if would be possible to have both at same time as third option, or atleast having dedicated hotkey to change it at instant, instead needed going to right panel ship settings.