The idea behind a cat delete is that you replace the catalytic converter with a section of straight pipe. You would either do this for performance reasons, or because catalytic converter theft is remarkably high in your area.

Deleting your catalytic converter releases more toxic fumes in the exhaust that are not good to breathe in. When done correctly, the cat delete still releases these fumes at the back of the car. Risk of exhaust gases entering the vehicle should be no greater than a factory exhaust system.