Most modern CPU's have 3 levels of caches, L1, L2, and L3. This way the processor doesn't need to keep going to RAM to get data, which causes a lot of additional latency. They also can't just make the L3 cache larger, because increasing it's size would also its increase distance from the logic, thus increasing latency. AMD solves this problem by stacking the L3 cache on top of the traditional silicon, and keeps the latency low by having it physically close to the logic. Poorly optimized games, like Skyrim and Rust, benefit from the larger 3D cache because the CPU doesn't have to access RAM as much by caching more data at a low latency.