This is an attempt at implementing fast screen capture with using the asynchronous GPU readback API.
AsyncGPUReadback is one of the new experimental features of Unity 2018.1. It allows retrieving GPU data (textures and compute buffers) without introducing render pipeline stall due to synchronization.
It's useful when GPU readback is required but small amounts of latency is acceptable.
Note that the AsyncGPUReadback feature is still in its experimental phase and only supported on DX11/12 at them moment. Further platform support will be added in later releases.