There is a common misconception among website owners that once your website is built, you can set it and forget it. You can just let it sit and linger forever without having to touch it. Unfortunately, this is not the case.
Much like a car, you need to regularly have it serviced to ensure that is operating efficiently and fix any issues before they become critical problems.
Why do I have to do updates?
WordPress is under constant development. New features are being added. Bugs are being fixed. Security holes are being plugged.
This means that the code upon which WordPress is built is constantly being updated. Therefore, when WordPress is being updated, all of its component parts, including themes and plugins, need to be updated too.
A Quick Primer on WordPress Core, Themes, and Plugins
WordPress core is just that. It is the core set of files that make WordPress work as it does. It is built on a series of thousands of files that all work together in concert to bring you the WordPress experience.
Every single WordPress site needs to have a theme. A WordPress theme is a series of files that determine the look and feel of your website. It is essentially the website's design.
WordPress plugins add features and functionality to your website. WordPress can only do so much out of the box. However, it is built to be easily built upon. That's why there are thousands of WordPress plugins that allow you to make it do whatever you want.