Frame Stiffeners:

What are frame stiffeners? Frame stiffeners are thick plates of metal that get welded to the side of unibody frames to provide extra rigidity. Jeeps with "real" frames don't usually need to worry about these, and they are most popular on XJs.

What does that do? The unibodies used on some Jeeps like the XJ are somewhat flexible, and off-roading heavily can flex the frame back and forth and lead to cracks and splitting. By adding thick metal stiffeners, you armor the frame and resist flexing which helps to prevent damage.

Any other benefits? Since frame stiffeners add rigidity to the Jeep, it can make a big impact on road driving, making the Jeep feel a bit more "solid". Some Jeeps, even those with "real" frames will also use frame stiffeners to support heavier engine swaps or other modifications.