Brake ducts (found on the "Aero" page of the cars setup) are mainly to control the brake temperatures, but will also impact brake wear, internal core tyre temperatures and drag on the car (but only on a minimal level). Prioritise brake temperatures over all other factors.
Brake temperatures should ideally not exceed 650 degrees. Aim to get close to this value without exceeding it at the point the brakes become the hottest during the lap. Exceeding 650 degrees will have the brakes overheating, leading to increased wear and longer braking distances.
Higher duct values open the brake ducts more and increase the amount of cooling when off the brakes, and increase drag.
Lower duct values close the brake ducts, reducing the amount of cooling and holding more temperature in the brakes, and decrease drag.
Each incremental step is not a linear change. The change from brake duct 1, to brake duct 2 or 3 is larger, allowing for more temperature variance than the differences between brake duct 4 and 5.
The amount of cooling, even on the same brake duct values, is different between the front and the rear of the car. The front brake ducts have a greater cooling effect than the rear brake ducts.
Note:: Brake ducts can also be used to help control core tyre temperatures. Heat from the brake discs radiates into the rim of the tyre, which in turn heats the tyres core and increases temperature. In conditions where tyres are very hot, open brake ducts more to help reduce the amount of heat transfered into the tyres core and reduce their temperature slightly.