As with many other vocal effects, also growl is a spontaneous sound that singers produce due to an expressed emotion. It is a commonly used vocal effect in soul and blues music, for instance. One of the most commonly know growl users is probably Louis Armstrong with his legendary What A Wonderful World. Other singers who have used growl in their singing are Christina Aguilera, James Brown, Michael Jackson and Whitney Houston.
Growl Is Produced When the Epiglottis And the Arytenoid Cartilages Meet In the Larynx
Complete Vocal Institute states that the sound of growl is produced when the epiglottis moves downward to touch the arytenoids. When the arytenoids that are attached to the vocal folds in the back of the larynx touch the epiglottis, they get to vibrate against it.
Make sure to also read our post on another vocal effect: distortion.
References: Complete Vocal Institute