2008/7/26 Platonides Platonides@gmail.com:
New users often want to *upload new files*, not modify current images. They are also often the most clueless, so not letting them change existing images until autoconfirmed is a good idea. Specially because that avoids vandals creating new accounts on commons and replacing with penis images the ones on article X.
It also helps discourage accidental overwriting, which used to be reasonably common - someone would upload something with a fairly generic filename, not realise they were overwriting an existing image, and we'd realise a few days later that the Belgian prime minister's article on several projects was displaying a large photograph of a train.