This plugin adds an improved database layer to WordPress. It allows you to do more then the default one and is always up-to-date with the changes core makes.
mysql_* functions
The mysql_* functions are officially deprecated for PHP 5.5 and are throwing E_DEPRECATED errors.
On https://core.trac.wordpress.org/ticket/21663 there is discussion on this topic.
This plugin reflects those discussions.
Why should I use this plugin?
You should use this plugin if you want to start using PDO / MySQLi for WordPress.