Hi Joe,
You do not need to buy a Backup CD. At any time in the future, you can download the same major version of the software you have purchased and use the authorization key that is emailed to you after your purchase to unlock it. However, it is a good idea to download the software and save it on your own backup media such as a CD, backup drive or thumb drive. All you have to do is click on the three installers and select 'Save' and it will be copied to your hard drive, most likely in your Downloads folder. From there you can back it up however you like. Whenever this installer is invoked, it will install the software in the trial mode and your code will unlock it. So even though you can look up your authorization code online, you should keep a copy of your authorization code in hard copy. I provide the Backup CD purely as a convenience and an easy way to do what few people do themselves.
When I talk about major versions, the current version of the software is V3 so any build that begins with 3 (3.0, 3.1, 3.11, etc.) is a free upgrade to anyone who has purchased v3. There are currently no plans in the works to create a version 4, but there will always be minor upgrades to version 3.
Please let me know if you have any questions.
Lloyd