Using a lock on a challenger basically, implies that you are having control of their limb(s) which might in turn control them. Martial Arts type locks utilize what is called pain compliance, and they are frequently used by people of authority to 'acquire compliance' e.g. Policemen, security personnel, security officers etc.
In First Action, Self Defense we hardly ever use locks to get control of others, however, they can be used as a great way to get away being held without using strikes. The beauty of utilizing locks is that they can be applied carefully or hard depending on the scenario. You can apply simply sufficient pressure to get away, or if you must 'disable' your opponent that can be done also just by applying additional force. Knowing the locks obviously means that they can be made use of for getting control of others if needed e.g. If it remains in your type of work, you should break up a fight amongst friends etc.