Definitely function, not looks. Trucks look better without mudflaps, but if you go off road or tow a boat / trailer much, you're probably gonna want good quality mudflaps to protect your paint jobs. All of the flaps mentioned here are good quality, but also all have their flaws. Depends on your particular usage and need. I use Weathertechs when I go off road or tow my TT, when I'm not doing either of those activities, I take them off.
Tim