Many websites are still running on older versions of PHP, usually PHP 7.4 or even older, and this can represent a security and performance risk for the website and therefore is always recommended to keep with the current version of PHP. For example, PHP 7.4 is already End Of Life to the 18 Nov 2021, this means that there are no security or code updates whatsoever. Someone could say that he do not care, which is an option, and since the website that runs the older version of PHP is not ciritical, even vulnurable security is acceptable. However, what if you would like to keep with Joomla 4? The recomended PHP version is 8.0 although the minimum still supported is PHP 7.2.5. This give us one more reason to not upgrade since you can still use the benefit of Joomla 4 on older PHP version but newer PHP means as well a performance boost and supported new features. Hence, it is more than recommended to move and upgrade the PHP to ensure the best exprience. Just look on the release note:
PHP 8.1 is a major update of the PHP language that brings better performance, better syntax, and improved type safety. It also contains new features and optimization improvements.