Deadlocks are locks that when in the deadlock mode, they deny a person from unlocking the lock from either side of the door without the use of a key or similar.
The idea is that a thief cannot open a door by breaking a window adjacent to the door and just stick their arm through the hole and unlock the door. If the door has a deadlock, they will still not be able to unlock the door unless they have a key. It also prevents a thief from being able to open the door from the inside if they have gained access through a broken window or something similar. This also prevents thieves from opening a door in order to remove large items.
Deadlocks can be dangerous in the case of an emergency where escaping a building needs to be done with the least amount of difficulty. In this case, it can be quite critical the time it takes to find the keys and unlock the door in order to escape. For this reason, deadlocks are illegal in most countries on main doors of egress. For this reason, you need to consult a design professional and your insurance company.
Please note that this information is general in nature and laws or insurance requirements may need to be met.
Some common deadlocks are the basic double cylinder deadbolt lock shown below and euro style mortice locks with double cylinder
Some deadlocks can also have dual functions such as the Paradigm lock and the 001 deadlatch
Commercial mortice locks can also be converted to a deadlock when the lockable function is selected on both sides of the lock and a keyed cylinder is used on both sides of the lock.