What is an exhaust flex pipe and why do I have to replace it?-PART TWO

My mechanic told me I need to replace it after I went to him with a very loud roaring sound coming from the engine/under the car. He said it is the pipe that connects to the manifold.

Continued

Answer: could be allot of reasons depending on the age of the vehicle, it could be the exhaust donut and flange where the manifold connects to the flex pipe or it could be where the flex pipe connects with the collector at the bottom of the car and then starts to the back and if you hit a speed bump or a parking block in a parking lot then I would say the collector has been damaged.

Answer: Ask your mechanic to show you what he's talking about. My brother-in-law is a mechanic so I get him to show me things. When he can't I go into an auto parts store & ask them. The manifold is where your exhaust mounts to the engine. If this is busted you'll use more gas & dump more bad stuff into the air. In the state of Colorado you can also get a ticket. Auto Zone has GM service manuals on line but you have to go to their store for them to show you anything. That's the way it is here anyway. I also pick up lot of info from some of the car shows on TV, but that's better for a cathead like me. Good luck with your quest for car knowledge.

Answer: Aftermarket exhaust manufacturers such as HUANYU METAL HOSE can supply short lengths of flexi exhaust in various diameters. As long as there is one available in the right diameter for your Pass at, then any competent garage with welding equipment should be able to make an effective repair for a fraction of the price of replacing the Cat. We have done this many times at the garage I work at, to save a customer the expense of a new Cat.

Answer: It’s called an exhaust pipe when it rust out it makes a very loud sound and you do have to replace it. But the catch is how much he is charging you and the cost of the pipe. Flex pipe is bendable pipe you can shape it to where it easily fits the manifold.

Answer: Most of the above answers cover what it is. 
It's probably leaking exhaust gasses. 
Reasons for replacing it: 
1. Danger of carbon monoxide into the passenger compartment. 
2. Car won't pass inspection 
3. Can be noisy

Answer: Anytime you have an exhaust leak forward of the back seat, there is potential for exhaust gas to enter the driving compartment. Exhaust gas can cause sleepiness and enough concentration can cause death, not to mention the noise factor that you are already experiencing.