Advantage to a spring under conversion on a 4x4.

I have a lifted 2002 4x4 tacoma with stock rear leafs with a longer shackle and icon longer travel shocks with resi, I see most trucks with long travel rear with a spring under conversion.

What is the reason for doing that and why would it be an advantage?

 

more up travel, more travel like 18in. if you want 18 inches of travel with out a spring under you would be at like 10 inches of lift.

 

What all is fully involved with doing it and what parts work the best, I would consider it if it truly would be something that would look better, ride better and have a longer life on and off the trails.