1. Treat every firearm as if it were loaded
2. Never point a firearm at anything you are not willing and ready to destroy
3. Keep your finger outside the trigger guard until you are ready to shoot
4. Know your target and understand what is behind it
You can never be too safe
Why do you do them?
Maximize safety. Many people are injured or killed by negligent discharges.
When should you do them?
Every single time you handle any firearm. This cannot be done too often.