Rear spoilers/wings are intended for two purposes:
1) To force the rear end down at high speeds, like over 150mph, and/or
2) to delaminate the air flow at the rear of certain vehicles at high
speeds. This delamination is intended to prevent aerodynamic lift that may
destabilize the rear of the car.

Item #2 is the reason for that little lip spoiler below the rear window of
an Audi TT. A similar, self-raising spoiler at the rear of the Porsche 911
does the same thing. The Porsche's spoiler, in its home market of Germany,
rises up at something like 120mph, which is roughly when lift becomes
apparent. The spoiler was recalibrated for North America to rise at about
55-60mph, otherwise owners would never have the satisfaction of seeing it
rise. (Otherwise, what the hell did they pay all that money for?)

For any legal North American speeds, spoilers and wings perform no function
other than perceived esthetic improvement. Wings are becoming rarer and
rarer. It appears they were a fad, same as the jacked-up rear ends that
were common on '69 Cutlasses when I was younger.