GPU readback lets you get the outputs of your shaders so you can do any kind of further processing. Or it could even be the result of a process like texture baking. This process can be done both on the SceneViewExtension and Enqueue Render Command as well as asynchronously or with a forced wait on the GPU.
https://dev.epicgames.com/community/learning/tutorials/enjJ/save-shader-output-gpu-readback-unreal-engine-5-c-tutorial