For wallpaper creators
Playing on a laptop currently with a 17.3" screen and my resolution set to 1600×900 needless to say finding wallpapers in that resolution is kitten near impossible.
Mine is also 1600×900.
Ofcourse there are more resolution, but 1366×768 is still one of the most common used today. There is no reason not to make wallpapers in it especialy if they are making them even in 800×600.
The reason they don’t make them for that specific resolution is because it is 16×9 aspect ratio, meaning that 1920×1080 and 1600×900 scales down properly to that resolution.
http://en.wikipedia.org/wiki/Graphics_display_resolution
Since there are only 6 aspect ratios, they only really need to make 6 different wallpapers at a reasonable max resolution for each and they will scale down properly for those ratios.
(edited by Lil Puppy.5216)