The key release lets you disengage the motor and open the door by hand from outside. On a door with no other outside access it is the difference between getting your car out during an outage and not.
Signs it needs replacing
The release cable stretched or slack
The key turning without releasing the motor
A release that will not re-engage
Fitting notes
Fitted on automated doors, most often where the garage has no separate side entrance. The Eazy Lift Mark II carries a battery backup, so the key release is the second line rather than the first.