I'd like to throw a different perspective to the reply from Caselec -
From that point of view you ensure all circuits are 'off' should you intentionally disconnect the garage sub-pane. However, if you are a home workshop enthusiast (power equipment, milling machines, etc.)...it sure is the 'sh*ts' if you accidently blow the sub-panel OCPD at 10:00 PM some night while working with a piece of power equipment and ALL the lights go out as well! My experience is if someone wants a subpanel in a detached garage they intend on playing with 'toys' requiring some extra amps...I put a sticker on the light switch and sub letting folks know the circuit breaker for the lights is at the house.