What benefits does Nintendo gets releasing the NX a year and a half after announcement?

[Soundwave]

I think it's coming out next year because the 3DS needs to be replaced and NX is the replacement for 3DS primarily (it acting as a console replacement is just secondary).

They will formally announce it around March 2016, then unveil games and full hardware at E3 2016, launch in fall 2016 with Zelda.