Nothing is ever perfect. Even when things are first released and all seems well, it takes only the passage of time to turn up errors of some sort. A typical Linux distribution can contain hundreds of packages and potentially thousands of files. Although great care is taken to make sure things are as perfect as they can be, with those numbers, errors sometimes happen. Updates aren't always about errors, either. Sometimes package updates reflect enhancements to the current software.
The nice thing about problems with your Linux system is that those problems don't tend to hang around for a long time. Thousands of programmers worldwide are involved in developing and maintaining the Linux kernel and the associated packages that make up what we call a distribution. When something goes wrong or a security bug surfaces, it is fixed and released to the community. Getting and applying these updates to your system is something that you should be doing on a regular basis.
These updates and patches may be free, but there are other considerations. Specifically, you'll find that when you are doing system updates, your network connection is definitely an issue. A given update may include multiple packages, and some of these packages may be quite large. A slow modem connection may be too little for a major update. You could still perform the update, but you may be online for hours.