Great Question! The purpose is to increase mlieage. When we purchased this coach some 5 years ago it had 11,000 miles and it now has 53,000. It had an air restriction by Monaco's design and I have tinkered off and on with some success. I have been rewarded with one whole mile per gal. But I think I've hit my limit because I started at 6.6 to 6.8 and now 7.7 to 7.9. Cats do not sip fuel, so I started thinking Air in Air out. And again I don't think I can better it but I keep trying.