When you're 'done' with something (email, document, piece of paper, tool... any digital or physical object), the question to ask is:
"Where should I put this so I don't see it anymore but I can easily find it if I need it?"
The more 'done' objects stay in our field of view or could be stored in 2-5 different places, the more unorganized we are.
Organization is about 'out of sight, out of mind' AND 'everything has its place.'