@tsch my pacemaker was implanted for the treatment of what they refer to as "sick sinus syndrome", or sinus node dysfunction. That's basically when the cardiac sinus node, located in the atrium and which normally works to initiate each heartbeat doesn't function normally, resulting in a heart rate that is too slow, or pauses between heart beats. It's also associated with various atrial arrhythmias/tachycardias (SVTs, atrial tachycardia, atrial flutter and atrial fibrillation. I have a history of different types of atrial tachycardia, including atrial tachycardia, atrial flutter, and more recently, A-fib, and these are treated medically, but I received the pacemaker for symptomatic bradycardia as my heart rate would frequently go into the high 30's-low 40's. I was one of those lucky people whose sinus node dysfunction was of a tachy-brady type. I got the pacemaker to treat the bradycardia, but having the pacemaker also allowed for me to be treated with medication to treat the SVT, A-fib and A-flutter ( my multi-talented heart does all those!) without my heart rate going even lower than it would on its own. Fortunately it's all paroxysmal, and most episodes last only a few minutes, thanks to the medication ( rate control metoprolol tartrate 150 mg/day) I take, so I have never had or even considered an ablation of any kind.

A pacemaker will also allow a person with tachycardias to be treated with medication that otherwise would cause their heart rate to go too low as the pacemaker is set for a minimum heart rate ( usually around 60) and the heart won't go below that number.

Far as I know they don't implant pacemakers solely to treat A-fib, A-flutter, as there are medications that can be effective, and they seem to be encouraging people to undergo cardiac ablations, even as a primary treatment, to treat those arrhythmias.

As I understand it, the AV node ablation ( ablate and pace) is considered pretty much a last ditch effort to relieve the symptoms of persistent/permanent A-fib in a patient with this condition. The AV node is ablated so that the electrical signals from the atrium ( disordered as they are with the A-fib) no longer reach the ventricles- this eliminates the "rapid ventricular response" which would drive the heart rate way up. Since it's generally the ventricular response that the person feels, the patient reportedly is relieved of the symptoms associated with A-fib. The person still has A-fib, but doesn't feel it any longer. A pacemaker is needed since the AV node ablation destroys its function, so electrical signals no longer get through to the ventricles. The programmed pacemaker provides the electrical signals to the ventricle(s) so they beat normally.
I've been told, and read that A-flutter ablations ( these are not AV node ablations) are pretty successful and straight-forward in getting rid of the flutter. I doubt they'd do an AV node ablate and pace for A-flutter, but that is a conversation to have with an electrophysiologist.

I've included a link to an article that explains sick sinus node syndrome, and another on AV node ablation and pacemaker placement.
https://www.mayoclinic.org/diseases-conditions/sick-sinus-syndrome/symptoms-causes/syc-20377554
https://www.mayoclinic.org/tests-procedures/av-node-ablation/pyc-20384978