Every scene in Ecstatica is seen through a fixed camera. In Software mode the re-release renders those backgrounds live, on your CPU, at your screen resolution. It looks sharp, but it keeps the processor busy.
This DLC does that work in advance. All backgrounds have been rendered at 8K and ship as finished images, so you see the best version of every scene while your CPU has almost nothing to do. There is one cavaet though - these background take a LOT of space.