I find the ability to completely "close" my phone really appealing. It feels easier to ignore it, and the screens feel more protected to me.
I can also only describe the second benefit as "its like two monitors". I can watch TV on one and respond to messages/browse on another. I use a Note8+ already, so big screens for watching crap are already my preference though.
Every phone I've had with a case, grit collects betweeen the case and the back of the phone and scratches the phone up. I can imagine the same thing happening between the two screens if folded in my pocket.
Samsung has the Android W series phones which have a similar format.
Regarding the sibling comment, after the Droid 4 and the Motorola Photon Q, there has been a drought of physical keyboard landscape slider phones. Portrait sliders have been available for the Blackberry Priv and KeyOne/Key2, but aside from the soon-to-be-released Fxtec Pro1, not much landscape slider android phone action is going on.