What are the metal parts which are placed against circuit breaker switches used for?
Those are breaker locks. You can hold the breaker in the on position or raise the bar and shut it off holding them in the off position. I can't tell if they are lockable to qualify as a legal lockout device for "maintenance disconnect" purposes.

YES, the breaker WILL still trip even with the lock in place. Trip mechanisms are internal and will work without the handle moving. As soon as the lock is moved the handle would pop into the middle position. 

These are used for circuits that people should not be messing with, like alarms, smoke detectors, furnaces, emergency lighting, computers, etc.