In a nutshell, a chronograph lets you measure the speed that the bullet leaves the barrel. This information, in conjunction with data about the bullet you are firing and the environment you are shooting in, lets you start mapping out the expected flight path of the projectile.
Firearms Safety isn't just about knowing the 7 Basic Rules of Firearms Safety, nor is it about remembering just enough to pass the multi-choice questionnaire that is a requirement of getting a firearms license in New Zealand. It's about making a commitment to maintain a high standard of firearms handling and safety at all times. One of the keys to this, is learning and reinforcing good habits when handling firearms.