AED doesn't necessarily help people with an otherwise normal healthy heart that have a cardiac attack from an accident (or too many burgers) they only help people who get an irregular heart rhythm.
So they are more likely to be needed in a public place with a large cross-section of random people than a job site with otherwise healthy people who might have a heart attack.

The machines have a built in analysis system that can tell the state of the patient - they don't apply the shock and will tell you to do CPR if the heart has simply stopped. So are generally safe to use on anyone.