The rollcage is a ThinkPad design feature found principly in T series and some R series models starting with the T60 model. The rollcage is made of lightweight magnesium and offers a high strength to weight ratio - the bare rollcage shown below feels as light in the hand as perhaps an empty file folder.
Here is a rollcage from a 14" model shown on top of a 15" model T61p for visual comparison.
The purpose of the rollcage is to dramatically reduce system board flex that can weaken solder joints, especially on large chips (i.e. GPU, Southbridge, etc), and help protect many of the system's other internal components, while providing a rigid attachment point for system hinges, and outer plastic or composite covers.
Some models of ThinkPad also feature a magnesium rollcage in the lid assembly to help protect the LCD panel, and / or use outer shell / case components that are magnesium or composite.
When the systemboard and other components like fans, wifi cards, and outer case pieces are attached via screws to the rollcage, all of those screw locations create a type of tension that makes the entire assembly much stiffer, stronger, and more resiliant than any of the components by themselves.
When servicing a system, be sure to carefully start each screw by hand to ensure it is not cross threaded. Screws that are cross threaded and forced in with a powered driver will bind and will either strip the screw head, or, could cause some of the smaller mounting tabs on the roll cage to snap off, necesitating it's replacement and further disassembly of the system.