so why do we have a fat lip?

[ruski]

I mean the rear fender lip.

Did they just decide to stamp the fender panels like that because they felt like it or is there an underlying design justification, like make the rear fender more rigid?

[AkXb70]

Im pretty sure the rear fenders are part of the structual element on the car, and that area does seem to compress in an accident (from the pictures I've seen), so it could also be designed to absorb crash energy.

Thats just my assuptions. I could be wrong.

[itslow]

Ease of manufacture, resulting in lower manufacturing costs.