While its not nearly as crucial as sight, sound and touch, the sense of smell can be a powerful thing. Like with all evolutionary quirks, it is yet another thing we use to survey our environment, and to alert ourselves to potential threats. It can also evoke a keen sense of emotion, sometimes causing shelved memories—both good and bad—to come flooding back,
Essentially, smell is the result of chemical reactions. Ergo, what we smell depends on the nature of the chemical reactions that are taking place.
Despite the fact that its a near-perfect vacuum, outer space has a distinct odor (many different ones, actually—most repugnant). Of course, you can't smell them outright (you would die), but you CAN smell the residuals, mostly what clings to an astronaut after they are safely inside the ISS.
In a new production, the folks from the American Chemical Society created a video that recounts the smell of space, as told by people who actually saw the sights and smelled the smells.