A failing car manufacturer in a failing city can't produce luxury. When you're selling a luxury good and "failure" is the word most highly associated with your brand, things aren't looking up. The media has linked Chrysler, Detroit, and economic downturn together in the minds of the American public ever since their first bailout in 2008. So in their Super Bowl ad, Chrysler had to find an approach that would manipulate not only peoples' perceptions of their brand, but their perceptions of Detroit.
Eminem has the perfect image to use as a form of higher classical conditioning. Since it would be a stretch to directly associate Detroit with luxury, they can make an association to it indirectly through Eminem. He's a powerful rapper out of Detroit, who represents fame and fortune, but is made edgier and even cooler by his gritty history. When he says "this is the motor city, and this is what we do", both Detroit and Chrysler get to ride on the coattails of his success and status.
Anecdotal reasoning also helps to shape the viewers' perceptions. When the voiceover says "It's the hottest fires that make the hardest steel" over images of steel fists and beaten city, although the phrase itself doesn't make all that much sense, it still provokes feelings of being a tough underdog. Chrysler's new car as a result seems pretty bad-ass.