People in Japan sometimes don't lock their bikes, or use very light, easily breakable locks.
The difference being that in Japan a symbol of ownership is usually enough to deter thieves, but in the UK you must physically stop people being able to take your bike away.
In the UK you use a lock for exactly the same purpose. All bike locks can be broken, however, if you use a lock that will increase the time that it takes a thief to steal a bike, thereby increasing the likelihood of them getting caught they will, hopefully, not try to steal your bike and move on to an easier target. You are still using the lock to deter thieves though.
@Bobbo
In the UK you use a lock for exactly the same purpose. All bike locks can be broken, however, if you use a lock that will increase the time that it takes a thief to steal a bike, thereby increasing the likelihood of them getting caught they will, hopefully, not try to steal your bike and move on to an easier target. You are still using the lock to deter thieves though.