The round tip reinforces the air cavity, creates a little swirl, clings approximately the body and adds clouds. Car designers explain that the rear lights of the air sculpture shows how it is worth making small improvements here.
Especially with EVS, where air dynamics plays a greater role in the total efficiency compared to ice cars, and because more clouds not only adds the cost of fuel, but also charges shipping time.
They are small details with the old A2. The door handles are closed, the windshield wiper sits over the edge of the windshield and there is a bodybone (very hidden).
Practical application and regulations played their role as well. The A2 contains a small lips spoiler its opening, which may be more efficient if it is larger, and it can be vertically stretched on both sides of the car, such as those they put on the back of the trucks.
But this would make the back vision worse than it can and can spoil the lines of what I think is a very small car. As if the rear lights have been carved, instead of forming them to match the body: more efficient and less attractive.
There are things that Audi did not do at the time, then, that she will feel almost had to do today, especially on a car being marketed, as A2 was, technician and effective.