In all the talk of graphical downgrades no one seems much preoccupied with 'why?'. Why build something and then proceed to tear it down, piece by piece, in the hope that ever more diminished expectations about the final product won't be severe enough to...
Shoot everything at least once. That's how you earn the big bucks.
Clearing waves gives you bonuses, so try and kill everyone within the time limit.
Shoot barrels when bad guys are near them for quick explosive kills.