Is there any reason why PHP files require write permissions?

Is there any reason why PHP files would require write access? (Excluding directories that need to be writable) I'm installing Magento via composer. They recommend the following permissions: cd /var/www/html/<magento install directory> find var generated vendor pub/static pub/media app/etc -type f -exec chmod g+w {} + find var generated vendor pub/static pub/media app/etc -type d -exec chmod