It's not basically 1080p, it's literally 2x more pixels, which is important for cropping to 1080p in select zones, or emulating lense zoom, stabilization etc.
Heh, this is where video format naming is stupid. 1080p = 1920x1080. 2K = 2048x1080. 2K and 1080p are practically the same. 4K = 3840 x 2160, which ends up being 4x the total number of pixels as 2K/1080p.