What causes your catalytic converter to go out on a 2005 Buick Lacrosse?

Thank you for the information. .....Bought the car used in December. Noticed some resistance when accelerating and burning alot of gas. Didn't know if they were related, was told at the car lot today that I needed the converter and it was ok to drive until part arrived. Was trying to get some understanding, it's all so overwhelming. I wanted to know if it could end up being an engine issue.