Visual editor not working Locally

Visual editor was showing up blank on all my local WordPress installs.
Running MAMP & Mixture on my Mac.

After paying about I edited wp-config.php and changed the following:

define('WP_DEBUG', false);

to

define('WP_DEBUG', true);

An KABOOM! the visual editor now shows up. I still don’t understand why this works and would be grateful if anyone could shed light on this?

Still though I would post this as may help others. ??