Regular PHP installation on a web server runs as the user nobody and it doesn’t require the execute flag to be enabled. The problem with regular PHP installation is that if mod_openbasedir is not installed, every user on the server will be able to read your php files because practically everyone shares the same username (nobody).
PHP Files are not meant to be read, but parsed, otherwise everyone who is able to read your php file will able to view settings that you would want to keep private, such as your MySQL username and password.
suPHP fixes this issue because it requires PHP scripts to be executed with the permissions of their owners. suPHP also fixes common file ownership issues that mostly occur with few Content Management Systems such as Joomla and also on the popular blog software: WordPress.