This style of locking bag uses an external padlock for locking the bag.  It is simplistic yet very effective.  When you close the bag the pull/zipper tongue has a small metal tooth that goes between the teeth on the zipper chain.  When you place the padlock through the metal loop on the slider body it locks the pull down which keeps the metal tooth in between the zipper teeth locking the bag.

This type of bag has distinct advantages over more costly locking bags, lower cost being one of them.  You can just change/switch or discard the padlock when you need to change the lock which can arise from things like personnel change or lost keys.  More costly bags, which have the lock as a part of the zipper itself, would force you to replace the entire bag or would have to be sent out to be re-keyed.  This would be far more costly then just replacing the padlock, which can be done in seconds.

This bag also allows you to place any type of plastic or wire safety seal/strap through the loop, which would also prevent the bag from being opened if that is what you prefer to use instead of a padlock.  That is also another definite advantage over bags that have the lock mechanism right on or at the end of the zipper.

While no bag is theft or pilfer proof, being able to lock the bag is certainly a deterrent and helps keep the honest people honest.