In this case are animated wallpapers, not screensavers.
You can do your own wallpapers also, even you can use a youtube video as wallpaper, or any video, or you can just download a wallpapers of the thousands avaliable...(some of them are uploaded by the steam community).
When is running, consume 0% of my i7 8700, 23 mbytes of ram, and 0.3% of my GPU ( 1080ti ) . But if you load any application on fullscreen, the engine enter in stand by mode so it free the resources.
The wallpapers are just amazing. Some of them have audio detection, its a very well programmed application.
I pay for it the equivalent of u$s1 ... so its cheap. Worth it.