Nobody likes to leave their belongings unattended. And everything on a motorcycle appears to be very accessible, with tank bags, side bags and saddle bags left on the bike while you step away for a minute.
One solution is to take everything off the bike and carry it with you, but that's not really practical, is it? It is better to keep everything locked, if possible. A simple, dedicated luggage lock is both compact and easy to store, and sufficiently strong.
Of course, even a good luggage lock won't deter all thieves and cannot offer a 100% guarantee. On the other hand, the outer material of motorcycle luggage is generally pretty sturdy, motorcycles are often parked in plain sight and a visible lock is an extra deterrent. Deterrence is also a form of security.
A luggage lock can double as a perfect way to secure a helmet or other accessory to your motorcycle.