You may not ever find a reason ro break down a door, but if the need arises, you know how to now. You never know when someone might become trapped in a burning building or when you're running away from the police.
Many seem to think that using your shoulder is the best way, trust me, don't do that - you'll end up hurting yourself pretty bad unless you're the hulk.
Instead, as the illustration shows, you need to kick the door near the lock, not on the lock itself. You of course also need to have good balance by digging your other foot into ground and having it behind you when kicking, so say it doesn't go down first time and springs back, you'll end up losing balance and going to the floor.