When you purchase XenForo you will get the latest version and will be entitled to updates for 1 year. After that you can renew for a further year. Prices are on the purchase page.
It is always recommended that you use the latest stable version of XenForo.
One of the few uses I can think of would be to know which nulled version to try and grab.
I know that's one of the first things that came to my mind. When you get a license you have an option to download the latest stable or the BETA (if their is an active BETA).