What sort of protection does a 2 pole circuit breaker offer?
Breaking both lines offers more safety during hands-on service of equipment in case the Neutral V is 10% of line voltage from heavy loads on other breakers. which is the worst case as I recall for residential wiring.

It also protects users from faulty reversed lines from inexperienced previous owners or contractors without a license.  It happened to me once with our previous house that had 50 pot-lights added to the house and when I was installing a 2way stair switch I found wiring errors. Buyer beware, even inspectors can't check everything. It took me a week to sort out the wiring in the attic with all the potlight ceiling wiring buried in blown insulation.  The risk of shock could be reduced with a double breaker here, but in North America, it doesn't happen unless its a 2 line 240V appliance. But as it happened when I saw the spark after removing the stair switch ground wire, I knew what to expect. So a double breaker would not have been useful as the symptom would have been masked. THe switch was connected between Hot and ground and Neutral was wired to the switch box.  sigh.