I understand people wanting to share skins or icons or software and other such things, but not scans of sketches or random art. And that's what makes deviantART so revolutionary. It's not an obvious concept (at least not to me).
Oh, I don't think it's such an obscure desire, to share artwork. Many artists keep sketchbooks that they not only use for archival purposes, but also to share with others (whether that be as a portfolio for a potential employer, or as a plain-old sketchbook to show to friends and family). The digital age just makes it easier to do so.
DeviantArt is interesting because it is so well-designed as a site. It's so easy to browse and to use. Contrast that with, for example, sites like conceptart.org. ConceptArt is a great site for artists, but the organization is atrocious. It's hard enough to see what's new on that site, let alone search for something you saw last week. Photo.net, arguably the largest photography-sharing/critiquing site, is better, but still not that great.
What DeviantArt did right was to combine the best elements of static pages, personal galleries, forums, wikis, and "social computing."