What 1296×864 is for
1296 × 864 crosses the one-megapixel line while staying in a 3:2 frame. It is 1.5 times 864, which makes it a tidy intermediate step when you are building a set of image widths.
This is around the size where an image can serve both as a content image on desktop and as a small print, since it holds enough pixels for a postcard-sized reproduction.