Ditto re: condensation. Ever notice how much steam comes out of your (or any) exhaust pipe when the engine is cold? You may even see water dripping from the end of your tailpipe while its warming up. Once the exhaust system is thoroughly warm it goes away. If that water just laid in there it would rust out a muffler in no time. Before stainless steel exhaust pipes you'd only get 3-4 years out of an exhaust system before you started replacing parts.