Simple answer: Keep a copy of all your purchases on a local drive even if it an archive drive you don't normally keep plugged in. Do not put yourself in a position where you have to re-download past purchases. Many you can re-download multiple times but Apple itself observes they may not always be available for re-download (http://support.apple.com/kb/ht2519).
iTunes Store: Associating a device or computer to your Apple ID - http://support.apple.com/kb/HT4627 - In connection with, "When you turn on iTunes Match or Automatic Downloads, or when you download past purchases on an iOS device or computer, that device or computer becomes associated with your Apple ID." "Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID. Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days."
Why all the above? No official statement but one could forward the argument it's a step to preventing media piracy.