I’ve had a few different First Aid courses and the instructors all have slightly different reasoning. One argument for compression only is potential for passing disease mouth to mouth, the newer courses tend to teach this because sometimes people that don’t feel comfortable doing rescue breaths will fail to do CPR at all. Another is that in cases where you’ve witnessed the event, the blood is already fairly well oxygenated and if medical help has a good response time the benefits of breaths are minimal. The first is more about compression only CPR being better than nothing, breaths are still advised where the rescuer feels comfortable doing so. The second is pretty situational.