Boosting the attack in Full Range mode can sometimes result in some pretty huge level peaks. These can make your processed signal sound uneven, and in the worst case they will create the nasty kind of distortion you don't want.
The nvelope solves this very conveniently with the Auto Gain function. This simple button makes the impulse shaper automatically compensate stronger level peaks, so this can be the solution for many level-related issues caused by the attack processing.
By the way, Auto Gain is typically not needed in Dual Band operation and therefore not offered in this mode. Also, Auto Gain does not have any influence on a channel operating in EQ mode.