Hello, I'm Krawper, the developer of this asset.
Most of these backgrounds are indeed already available on the site, but here we offer a version where all 280 scenes are ready to use with the Unity game engine (complete with scripts simulating the parallax effect and a default player for testing). This saves a huge amount of time by eliminating the need to configure everything yourself.
I hope this clarifies things for you, and please excuse my very late reply.