There are 3 main reasons for upgrading WordPress. The first, and probably most obvious reason is so you can enjoy the latest and greatest new features and upgrades to the software.
The Second reason to stay current with your WordPress installation is so that you can install or continue to use the latest versions of your favourite plugins. As new versions of WordPress are introduced actively updated plugins are modified to work with the latest releases of WordPress. If you get too far behind in your WordPress updates you run the risk of not being able to run the latest release of your favorite plugins.
While the first two reasons are important. What if you’re happy with the way the site currently functions? What if you don’t have any need for any of the new features in the latest version? If it ain't broke, why fix it? In a word… Security. All content management systems, or any website for that matter, are vulnerable to hackers. Since WordPress is currently the most popular CMS today it’s important to keep it updated as it can be a target for hackers. The good thing is that WordPress with it’s large user base and active development community quickly issues new versions of the software when security flaws are discovered. In fact, most cases of hacked WordPress websites are due to not keeping up with installing the latest release. For this reason it’s important to keep your WordPress site up to date and to keep your website data regularly backed up. This is especially true if you are continually adding content to your site such as with a blog site.