the answer to why would be to improve performance and to remove "unnecessary" scenery in favor of being able to make "necessary" textures look better/perform better.
for instance running car reflections on medium so that the mclarens look ok (try setting that to low and looking at one of them, they're hideous on low) but disabling mirrors.
I tried to do this myself but the tweaks I made didn't help anything/change anything. also setting the hardware_settings_config.xml to read only after I edited forced the game to crash.