To fully answer your question, the PIN is a unique number assigned to a specific BB device. Each device has its own unique number. Think of it as the device ID or the device's individual email address. This is how RIM determines where to route the email for that BIS/BES account. Communication via PIN to PIN is just an additional benefit of the RIM implementation.

If you buy a BB from ebay or from someone, you might get their email because their BIS account was not closed and RIM's servers don't know the device now belongs to someone else.