the coil is for added support ( think a bra) for when you are loaded up a bit  or running soft springs or both.  Leaf under.. well if you have to ask why you dont know a whole lot about suspension lol.. sprung under just has way more control, less axle wrap, better at climbing i could go on. and such big springs.. well when you actually build a real leaf pack they have more than the 4-5 leafs that the majority of people run.. think more in the 7-8 leaf range, aswell as being sprung under he has to run a little bit more of a taller spring so to speak to get the lift.