As a HCP who has performed CPR on a lot of elderly or frail patients, I can give you a brutally honest view of what it's like and why it's never, ever appropriate. If you're not up for that and you'll find it upsetting, don't read the rest, just read this and the last paragraph.
In my entire career and that of my colleagues, I've never successfully revived a patient over 80 and over 70, I've never known anyone get back to normal health.
I'd say about 85% of cases over 70 but under 80 who have had CPR I've known have died within the year and probably 60% of all CPR patients over 70 have suffered some kind of lasting injury from the CPR itself.
In the over 80's, CPR is utterly traumatic. At that point it's very very easy to break ribs/ cause internal bleeding through CPR. Hearing ribs crack as you're doing compressions is horrific, but you have to continue doing compressions which you KNOW is causing further damage the whole time but there's nothing you can do.
When HCP's talk about someone 'coming back' on TV, you often picture someone waking up and coughing then thanking everyone etc etc. This does not happen. Ever. Most of the time, a successful CPR means regaining a heart beat, NOT the patient waking up and being well. Depending how long a patient is down for, the brain is starved of o2 which causes brain cells to die off very very quickly. In my experience, the most common post-CPR complications are brain function. Bones will heal (albeit incredibly slowly) but brains at that age dont.
Even when your patient is young and fit, CPR is horrific. To do it to a man who is already confused and frail is, in my opinion, cruel.
Sorry if that was a bit blunt. So the benefits of a DNAR are that you won't subject your father to an incredibly brutal process at his most vulnerable time, where the likelihood of it's success in allowing him to live a longer life with no consequences is close to 0%. The likely outlook if he had CPR and recovered is spending his last few hours/ days/ months increasingly confused, in pain and probably in hospital for the majority of it. Quality of life post-CPR for the elderly is not positive.
You should be talking this through with him, it's his decision ultimately if he's well enough to understand but as a family, I wouldn't suggest encouraging him.