Here are the three important reasons to keep your PHP version updated.
Better security
The key reason to constantly keep your PHP version updated to the latest is for security. Unfortunately, older PHP versions are no longer getting security fixes, meaning known vulnerabilities are not fixed on that version, leaving your site open to attacks. In other words, newer versions are better at countering hackers.
Faster websites
Newer PHP versions will execute code faster, which means faster page load speeds. Faster page load speed means better user experience and good SEO signals. As a result, WordPress sites will benefit significantly.
New and improved features
As with all updates, a newer version includes new features and improvements on how to do things. For example, preloading.