For some people it's less eye straining if they're reading at night, doesn't help for me personally, but it's a popular feature

Because most of the page is dark, with light only coming from the text, the total light emitted is much less. That makes it less annoying to my wife, when I'm reading before going to sleep.

I have no idea but users seem to like it despite all the ghosting.

The reason I use koreader is because it can sync my progress on disparate devices. I have a normal sized kobo and also a phone sized viwoods eink device.