I must admit, generally when I submit a ss I suppose I'm showing off a bit, but just because I think my dt is the coolest thing since sliced bread doesn't mean the next person will think that. I also submit my ss to promote interest in my upcoming next set of skins or new features in HVD.
I view other people's ss for inspiration with new projects, and just general nosiness!!
I think screenshots are here just to share your dektop with others. Sometimes i see a desktop and i want exactly the same. I also think, if you submit a ss, please write down what programs/skins you've used. If you don't you're just showing of. It should be about SHARING.
It came from the use of shells [other than exploder]...and has been around for a long time to show themes/shells in action where some people may not be running the same system and cannot replicate it themselves. OK, so if some dick-weed loads his brand spanking new XP default setup and opens a winamp skin and says 'woopy-do, look at me ain't I a wanker?'...we can all agree.
As I remember, one of the most interesting sections to keep an eye on at skinz.org was the screenshot section....but then, I'm a 'STEPper, not a wallpaper spammer....;p