Just because a person is cordial enough to knock at your door doesn't mean that he isn't a conman or burglar out to knock you out and steal all your belongings. So you can probably understand the usefulness of a spy hole, or a door viewer.
A door viewer allows you to look at the person knocking or ringing your doorbell first before opening the door. Check out this video to learn how to install a door viewer.
This video shows how to quickly install a door viewer (often called a spy hole) into a solid wood door.