Why you would need this:
Example 1: Hostels
As a couple we do not share rooms with other people, meaning no bunk beds in Hostels, but if you want the cheapest accommodations, and a great way to meet people from all over the world, Hostels are it! …then you might need this.
Example 2: Trains
While on a long train ride in Italy, we were both falling asleep and our bags were in the overhead bin across the aisle. We always stand up and watch our bags at every stop. It is very easy for someone to grab your bag and jump off a train at a stop without you having time to react before the train takes off again. What do you do? get off the train, possibly leaving your other stuff or travel partner to chase the villain? No, that would be silly. Far smarter to keep your bags right with you. Connected in some way to your body, preferably. Or, as with this device, locked to the railing of the train.
NOTE: This is why we do not travel on busses for long trips. Busses usually make you put your luggage UNDER the bus, where you can no longer keep your hand on it. If the bus makes multiple stops, anyone who gets off at any stop can claim your luggage is theirs, by mistake or not!