Obligatory folklore.org [1]... design is often how to incorporate what we already know and see in the digital world... regarding your comment about folders, if you do a google image search for "folder" you'll find most of them are rounded. In drawer at home, most of the folders have rounded edges where possible (ie, not at the hinge of the folder due to increased tear potential).
' Steve suddenly got more intense. "Rectangles with rounded corners are everywhere! Just look around this room!". And sure enough, there were lots of them, like the whiteboard and some of the desks and tables. Then he pointed out the window. "And look outside, there's even more, practically everywhere you look!". He even persuaded Bill to take a quick walk around the block with him, pointing out every rectangle with rounded corners that he could find. '
' Steve suddenly got more intense. "Rectangles with rounded corners are everywhere! Just look around this room!". And sure enough, there were lots of them, like the whiteboard and some of the desks and tables. Then he pointed out the window. "And look outside, there's even more, practically everywhere you look!". He even persuaded Bill to take a quick walk around the block with him, pointing out every rectangle with rounded corners that he could find. '
[1] http://www.folklore.org/StoryView.py?story=Round_Rects_Are_E...