Look under your Jeep at the suspension. You'll see something like this...
Your leaf springs are bolted straight to the chassis at one end. They are simply allowed to rotate on the bearing.
As you drive along, your springs flex. This makes them longer and shorter as they do so. So at one end of the spring, a shackle (The thing at the end of the spring in the picture) is fitted to allow the spring to extend and contract. As you hit bumps, the shackle sways from left to right (as shown in the pic).
Shackles also have the benefit of raising the chassis and body. This is what will give you clearance for bigger tyres.
