The stock truck has rake to it, or a forward lean. Many don't like this. To correct it, you can either add preload with collars or add longer springs. Springs cost more and are harder to install, but work better.
The rear has 2" blocks stock, so +2 springs remain at stock height, +3 springs raise it an inch, as they remove the 2" block.
You don't need both, it is all preference. Some do just front, some just do rears.
Personally I have 2.25" collars up front and +3 deavers in the rear. I do not have an unlimited budget and I do my own work.
If you really want to ball out, get some fox 3.0s which are adjustable.
Hope this helps.