Since the advent of disc brakes on road bikes, the designs have adapted to the needs of road cyclists. Flat mount is one of these adaptations. They help to position the brake closer in for better aesthetics and aerodynamics. Also, the forces of the rear disc brake route into the chain stay, which is stronger than the seat stay. They can be more challenging to set up, and there is not as much wiggle room for caliper positioning as with post mount brakes or I/S mount, but those differences are small and worth the sacrifices.