[afiori]

Since the door/lock analogy is widely used I would like to point that they have 2 other major advantages

1: they have the effect of warning you of unauthorized access: you can maybe break a door/window in a matter of minutes but you wont catch me napping nor you will be able to make it look like nobody broke in.

2: they keep unmotivated attackers out and can move the attacks off to you and on less protected properties.

[marcosdumay]

Do we actually know if it has any effect at all?

Locking your door looks more like an historical artifact from the time the police wasn't ready available for policing the neighborhood of normal people.