1) Simplicity. Mysqli is object oriented. It's simple to use. You create an object and pass your database information to the constructor. And you're all set.
$mysqli = new Mysqli("localhost", "username", "password", "database");
$query = $mysqli->query("YOUR QUERY");
$row = $query->fetch_object();
echo $row->result;
2) Speed. Mysqli is newer and runs faster than Mysql.
3) Cleanliness and Organization. You will be more organized using Mysqli, Mysql is cluttered.
I highly recommend making the switch to Mysqli, you won't regret it.