Let's be honest, people do fart, often in public. It's a shame for them and a torture to the public. Deodorizing underwear would help farters feel more self-confident and those around them happier.
Well, it's also a renewable energy source. This energy should not be wasted. A micro power plant could be built in in the underwear. The energy produced could be used for recharging a mobile phone. This would be especially useful when you travel in remote areas, say in mountains.