I've always thought that smells were one of the strongest indicators of place -- the beach smells of tangy salt; my high school smelled, oddly, of undercooked potatoes; certain streets in my neighborhood are overwhelmed, at the moment, with blooming jasmine. There's something psychological about it, and indeed it's been proven that smells can stimulate memory in astounding ways. So why not map the world's smells?
Sure, it's pretty subjective -- something that smells like greasy burgers to me might suggest another scent to you, but it's interesting to see where people's perceptions overlap. Also, I can think of some situations in which this app would actually be fairly useful, like that five-mile stretch of highway that passes a cattle feed lot, the smell of which invades every inch of your car no matter if your vents are closed or not. With Stinky Maps, you might think to plan a detour!
Searching around the Stinky Map on my iPhone, I find spots that people have listed as smelling like rosebushes, cheese, manure, cut grass, wet asphalt, and even a store that employs someone with bad breath. As for the app's practical usefulness, well, it hasn't exactly come in handy yet, but I'd say 99 cents is a fair price to pay for the novelty of discovering what other people think my city smells like.