Used to keep thieves out, or criminals inside, building locks are an essential thing in maintaining power in your location.
You must be inside the chosen building to add a lock to it. When the building lock is finished, the key to it will go to the project initiator's inventory. A key can be copied, with the use of 10 grams of iron and a file.
Please also note that this reference is in regards to a Building Lock, and not a vehicle lock.