Rear spring question

[drewzx3]

Inside of the rear springs there is this round rubber thing inside of it(not the isolater?. What is it? Do I need it or just leave it out.

 

[The Green GT]

Thats to keep the springs from bottoming out I think. They cannot compress any further than the rubber piece. My car has none since I lowered it.

 

[HISSIN50]

The little cylindrical rubber boot inside the coils is actually for NVH. It's a vibration damper. I swapped mine onto new rear springs but tons of folks have removed those and never noted any difference.