Nothing stays the same. Life is a constantly moving forward, and software moves forward faster than most things. vBulletin is constantly changing as new versions are released. Sometimes these are big changes (such as from version 2 to version 3), and other times they are just bug fixes and security updates (such as 3.5.2 to 3.5.4).
The updates to vBulletin generally contain security fixes rather than cosmetic tweaks, so it is important to install them in a timely fashion so as to eliminate security vulnerabilities in your discussion board.
Purchasing a license for vBulletin (either leased or owned) entitles you to a year's worth of updates. After this, you will need either to renew your license (if ...