Open back are exactly that. They are not sealed cups, the rear of the driver is exposed to the world and as a result have the ability to sound more natural and part of your environment. Not always though.
This can be a positive and a negative.
I personally use closed back cans for work in the office and at home when the kids are going nuts (when working from home) and when they are asleep I tend to use open backs on low volume so I dont wake them, but I can hear any action in the house.