Most manufacturers make both deadlocking and non deadlocking nightlatches,what's the difference and when should you use?

The difference?
 
Deadlocking nightlatches such as the Yale 85 can be locked from the outside by the key meaning the the handle on the inside will be locked and it will not retract the latch,this can also be known as double locking.

Non deadlocking nightlatches such as the Yale 91 by contrast cannot be double locked.

When to use a deadlocking nightlatch?
Once a nightlatch has been deadlocked the latch cannot be slipped using the credit card trick popularised in TV and Film and so provides a higher level of security.It can also prevent burgulars from smashing a glass panel and reaching in to unlatch.

Once the nightlatch has been deadlocked from the outside the internal handle will not open the door.Therefore people could be locked inside so deadlocking nightlatches should not be used on shared or communal houses.