WordPress is totally free because it is developed by a community of developers. Every time there is a new release, it will fix bugs, add new features, improve performance, and enhance existing features to stay up to date with new industry standards.
This means that when you do not update your WordPress website, you are risking your website being hacked and missing out on new features / improvements.
Click here for a list of WordPress recent and past vulnerabilities.
Hackers can made use of the vulnerabilities to gain access to your site to create havoc and make use of your site for spamming, steal information, criminal purpose, and the list goes on . . .