I am by no means a Suspension Guru
but others have called it spring cone isolators or bump stops and AFAIK will cater you approx. another 3/8 - 1/2" of rear lift.
I'm pretty sure all 3rd Gens came with them, some lifted truck owners choose to leave them in for the extra free lift(I did), some didn't.
All depending on the front vs. rear suspension set up and how you want your truck to sit. Metal to metal contact is the other issue associated without it and from what I read- feels more like the springs are shifting during articulation,(e.g. over bumps etc...)causing some clanging noise.....enough to bother some!
Bottom is that 1/2" of extra lift is very negligible and if your the type that loads up your cargo, then that's another bonus.