If a VF victim gets his heart pattern restored immediately following a sudden cardiac arrest, he has about a 2/3rds chance of recovery. Every minute that revival is delayed, the chances drop until there is little hope after 10 minutes.

CPR is not a means to revival. CPR is a temporary bandaid to keep oxygen flowing to the brain until a way to revive the victim is ready. Not many years ago, that means of revival was only available in hospitals, operated by highly trained medical personnel. Manual external defibrillators are used to apply an electric shock to victims in an attempt to restore a useful pattern to the heart muscle. Doing CPR in the field while a means to revival is bolted to the floor in a hospital does the victim no good. 
The next step was to make the defibrillators portable. These portable defibrillators were installed in ambulances. With this advancement, CPR in the field made sense. If a person could keep blood flowing until the ambulance arrived, there was a chance of recovery. But, arrival times could be 15 minutes, 30 minutes, or longer.

AEDs Arrive   Around 1980, the AED (automated external defibrillator) debuted. The ‘automated’ part is what is really exciting. Now people with only minimal training can successfully revive VF victims in the field, even before Emergency Medical Services personnel arrive. An AED can be down the hall, just 1 or 2 minutes away, so chances for revival are way up. The AED can automatically analyze the victim and decide if a shock is required or not. The operator just needs to follow simple commands from the AED and push the shock button when/if told to push it.

AEDs are nearly error-free, weigh under 5 lbs, are simple to learn and operate, are safe, can be stored anywhere, and they work!   There are a few different types of AEDs and improvements are being made all the time. Many people have home defibrillators and there are personal AEDs that are worn under clothing and automatically analyze and apply shock as needed.