It is very important that you are running the latest version of WordPress. Its popularity ensures that it is a target for those attempting to break into web sites (for whatever reason) since any security hole found in WordPress will give them access to a huge number of sites and not just the one site that they would gain access to with a security hole found in a custom written site.
Unlike many other scripts it is trivially easy to keep WordPress up to date. WordPress started out like other scripts where it would be necessary to upload the new version of the script the same way that you had to upload the original version and then manually run a conversion script if the new version needed to make any changes to the way the data is structured. The same applied to all the plugins and templates that you might have installed.
Updating early versions of WordPress was so complicated that someone created a plugin to simplify the process of keeping your WordPress plugins up to date. With this plugin installed you would be automatically informed when newer versions of any of the plugins you have installed became available and provided with a link that would allow you to have the script retrieve that plugin and install it in place of the previous version for you. The script automatically took care of placing your site into maintenance mode while it replaced the plugin.
This plugin proved to be so useful that it was eventually incorporated into the core of WordPress itself and extended so that you can not only use it to keep your plugins up to date with a single click but also keep all your templates and WordPress itself up to date with a single click to update each component. Now there is even an "Updates" link at the top of the WordPress dashboard menu that will advise you if there is a newer version of WordPress or any of your plugins or themes available to install by displaying the number of uninstalled updates directly on the dashboard. Going to the updates page will provide you with links to individually update each of them.
This update facility gets the new version from the appropriate web site for you automatically and installs it directly into your live site. You no longer need to go to the appropriate site to download the updates and then manually upload them to your site. You no longer need to read the instructions on what you have to run to be able to update from one specific version to another - all you need to do is to click a link and wait for one specific update to complete before you click on the link to start the next.
This makes keeping WordPress up to date so easy that anyone using it can apply each update as soon as it becomes available regardless of their level of knowledge of how to interact directly with the files on their web site. This means that unlike with other scripts where the site owner may end up many versions behind the latest one with a potentially vulnerable script simply because they don't know the first thing about how to update their script (or even that a newer version is available to update it), Those using Wordpress can easily keep their scripts entirely up to date simply because the dashboard tells them when updates are available and provides a simple way to automatically install the updates.