It is a sturdy wooden door. It is a house. The theory (suggested by a locksmith) is that a burglar breaks in. Then takes their time getting stuff out using the door (as if he's a resident or a mover) without anybody noticing. Pls see picture.
Yes, that's quite true. It is supposed to make burglars uneasy if they can't get out in a hurry (hence spare keys should be locked away) and also if they have to climb out through a window, they can only carry away what will fit in their pockets.
As you have a wooden door, you can easily fit a deadlocking latch.
I have one.
Entry from outside always needs a key
Exit from inside (or opening the door to visitors) can be done without a key
It can be deadlocked so the door can no longer be opened from inside (if you know for certain the house is unoccupied, e.g. all occupants go to work, or you go on holiday).
Traditionally, UK homes often had a yale lock, and a separate deadlock lower down, but people get into the habit of not using the deadlock.
If there is a risk of occupants being locked in the house, you can get one without deadlocking. With a wooden door you should protect the letterbox and preferably use laminated glass if a glazed door.
To defeat small children, they are often fitted quite high, out of reach.
As you have a house, you probably have a back door or other means of emergency exit.