CPR alone usually wont bring someone back, but what it does is buy time for them to get more advanced treatment that might.

There’s some exceptions, things like asphyxia and drowning have a pretty decent chance of bringing the person back if it’s done properly and promptly. In things like opioid overdoses it can buy you a couple critical minutes for cops or the ambulance to get there and shoot some narcan up their nose and then they’re back on their feet in no time flat.