Farts smell because of gaseous compounds that are either created in the gut or released as food is digested. For example, onions release sulfur compounds during digestion that create onion-smelling farts.

Because of the methane and sulfur dioxide.

The odor of farts comes from small amounts of hydrogen sulfide gas and mercaptans in the mixture. These compounds contain sulfur. Nitrogen-rich compounds such as skatole and indole also add to the stench of farts. The more sulfur-rich your diet, the more sulfides and mercaptans will be produced by the bacteria in your guts, and the more your farts will stink. Foods such as cauliflower, eggs and meat are notorious for producing smelly farts, whereas beans produce large amounts of not particularly stinky farts. Taken from the fart facts page at http://www.heptune.com/farts.html

So the Deaf will have Fair Warning.

The potency of a fart depends on the amounts of hydrogen sulfide and mercaptans (a group of sulfur-containing organic chemical substances).

It's for when your in a dark room and need to know when someone's there...

"Nitrogen, the main constituent of air, is the primary gas released during flatulence, along with carbon dioxide which is present in higher quantities in those who drink carbonated beverages regularly. The lesser component gases methane and hydrogen are flammable, and so flatus containing adequate amounts of these can be ignited. However, not all humans produce flatus that contains methane. For example, in one study of the faeces of nine adults, only five of the samples contained archaea capable of producing methane. Similar results are found in samples of gas obtained from within the rectum. The gas released during a flatus event frequently has an unpleasant odor which mainly results from low molecular weight fatty acids such as butyric acid (rancid butter smell) and reduced sulphur compounds such as hydrogen sulphide (rotten egg smell) and carbonyl sulphide that are the result of protein breakdown. The incidence of odoriferous compounds in flatus increases from herbivores, such as cattle, through omnivores to carnivorous species, such as cats. Such odor can also be caused by the presence of large numbers of microflora bacteria and/or the presence of faeces in the rectum." Source and further information: http://en.wikipedia.org/wiki/Flatulence